Well, it kind of depends on what you mean by "final villain". Are we talking about "the last person we fight" or "the main threat of the story"?
Because, I honestly think Buggy has a good shot of being the last actual threat the Strawhats have to fight. But, he'd be more of a "epilogue villain". Like say, we fight everyone else, we return the Moon to it's rightful place inside the Hollow Earth, it looks like everyone's getting their happily ever after...and then Buggy shows up out of nowhere to stab Luffy in the back. Then the Strawhats have to deal with him after they're already wiped out from fighting everyone else. Or maybe we wait until the epilogue, where the Strawhats reunite for another journey together after splitting up for a bit. And as everyone shows up one by one to get back together like in Sabaody, everyone gets a chance to kick Buggy in the nuts (literally or metaphorically) one last time for old time's sake. Buggy would be more like Saruman taking over the Shire in Lord of the Rings. Not the "main" threat, but the last loose end we need to tie up.
...If you're curious about that line about the Moon, look over here:
https://worstgen.alwaysdata.net/for...who-is-the-moon-god.68598/page-2#post-6140985
But, the spirit of the question really is "who will be the biggest threat in the story". And as for that, here's my read.
Let's start with some of the less likely candidates.
Akainu? No. I'm sorry Admiral fans, but I just don't see it happening, myself. If Hibari does turn out to be his daughter, that alone softens him a little too much to be the true final threat. Plus, we now know from Rock's monologue this chapter that one of the likely steps in Blackbeard/Rock's master plan is to crush the Marines. So, it's actually pretty possible that Blackbeard could be the one to kill Akainu and steal the Magma Fruit for his third Devil Fruit.
Shanks? Again, nah. I know it's been a bit of a meme that Shanks has been this 4D chessmaster manipulating everyone this whole time, but I don't see him actually turning out to be "evil". Maybe he's trying to make sure the World Government stays in control, just to maintain the status quo and keep chaos from breaking out all over the world, but he's not secretly a supervillain. I do expect Shanks and Luffy to eventually fight. Because this is a shonen battle manga, and you almost HAVE to have your protagonist fight their "mentor" at least once, it's manga law. It shows your main character's growth over the course of the series. But, that might be something we do at Lodestar or at the entrance to Laugh Tale, not at the very end.
Koby? Once again, not happening. I mean, like with Shanks, I do think Luffy and Koby eventually have to have some sort of a fight. Just because Koby's been here since the very beginning of the story. Or they need to team up, back to back, to fight a bigger threat together. But, Koby is not going to pull a Aizen on us. At least, I hope not.
Shamrock and/or Garling? Well...I could see one or both of them making a play. I could see there being a twist with this family. But, I don't think they'll end up overshadowing either of the two main candidates. I could see maybe Garling commandeering Uranos and going rogue during the big final battle. But, it would probably be rather short lived. And it might just happen to "help" the true final villain, accidentally. Garling accidentally ruining Luffy's attempt to deliver a final blow at the last minute by interrupting the batlte, that kind of thing.
Okay, that takes care of some of the major dark horse candidates. Let's finally talk about the Big Two.
We'll start with Imu. I honestly doubt that the final villain will turn out to be Imu. Because, ultimately I think Imu's going to turn out to be sympathetic. Imu is being portrayed as a literal devil right now. But, I think that's to contrast what we're going to learn about them later. Like the current theme song for the anime says, "What do you see? Am I a angel or a devil?" Oda is making Imu LOOK really elementally evil, so that when we learn their backstory, it hits us even harder. I honestly think Imu's going to turn out to be someone who's been trying to make the best out of a bad situation. A person who's had to make some tough calls, but has tried to do the right thing for the most part. A person who's made the wrong choices, but mostly understandably so. I like to imagine that Imu will turn out more nuanced than some people expect. Maybe I'll be wrong, but that's still my belief at the moment.
Because here's the thing. I think the Celestial Dragons eventually will have to get "redeemed" in some way. I know that feels distasteful at the moment. But, I really doubt that Oda wants his hero to kill off an entire group of people. That's just not going to happen. A core theme of this manga is that everyone, even the worst kind of people, deserve a second chance to try to correct the mistakes they've made in their lives. That's likely why Luffy rarely if ever kills his opponents. He doesn't want to kill them, he's knocking some sense into them. I think Luffy wants EVERYONE to be able to attend his big celebration party at the end of the story. Friends AND enemies alike. And yes, that's probably going to include the Celestial Dragons. So, either the Celestial Dragons have to get "redeemed"....or Imu and/or Blackbeard have to sacrifice them to use their lifeforce to power a giant superweapon, leaving Luffy out of the choice completely. A villain could kill the Celestial Dragons, but Luffy can't, just for the sake of the narrative. But, if the Celestial Dragons get "redeemed", then it makes sense for Imu to be front and center to show off that redemption. If we're "redeeming" the Celestial Dragons, it only makes sense to me if Imu also gets "redeemed", just as a prominent example of what that looks like.
And another thing that makes me think Imu will turn out to be not that bad of a person is that the Marines exist. The Marines are antagonists in this manga. And we've seen quite a few bad ones. But, for most of the world, they really are seen as heroes. Because, the Marines DO help "the common man". They help stop crime. They save people's lives. They're a part of their community. The Marines don't exist solely to help ensure the Celestial Dragon's rule. That's part of it, but it's not all that they are. And...Imu didn't have to do that. The Marines don't HAVE to exist in the way they do to ensure Imu stays in control. Imu created a force for good in this world. Even the Revolutionaries don't really want to get RID of the Marines, they just want to overthrow the Celestial Dragons. And I just don't see a force of "pure evil" building a organization like this or allowing it to continue to exist for hundreds of years. The existence of the Marines feels like proof that Imu has good intentions, at least.
There's also some other random reasons. Imu...is smol. Imu's seem really short, which seems like it could lend itself to a eventual sympathetic or even "cute" design once their face is revealed. Imu is hiding themselves from the world, which just doesn't strike me as something a "supervillain" would do, it just takes too much restraint, and most villains just love attention too much. Personally, I'm betting on Imu being the "Tree Dragon" from the Chapter 1138 Mural, and they likely have the Forest God Fruit. Imu lives above the Sunlight Tree Eve, they spend a bunch of time in the Flower Garden, they can "send forth Devils", everything points to Imu likely having Forest God powers. And if that's the case, it doesn't seem like "final villain" level powers. Thematically, it just doesn't make a lot of sense for a sea-faring story to have a Forest God as the main villain. Imu also seems like a bit of a scaredy-cat. If Imu trembling with fear after sensing Joyboy's Haki Knot and only leaving Pangaea Castle by "remote controlling" others is any indication. That just seems like it's setting up Imu to be sympathized with, when we learn WHY they're so afraid of leaving the castle. Imu's greatest failing is probably that they're a little too afraid to try and do something that needs to be done. And I'm personally expecting Mosa to turn out to be Imu, which would be Oda planting the seeds for Imu's redemption fairly early in the story.
So yeah, Imu's probably going to remain a threat for a while. But, ultimately I think we'll learn why things are the way they are, and Imu will turn out to not be that bad of a person, in the long run.
Now, Blackbeard. I think Blackbeard is going to be at least CLOSER to being the final villain than Imu is. While Imu might end up being portrayed in a sympathetic light, I could honestly see Blackbeard just ending up looking like a psychopath. Imu seems to have some sense of duty and decorum. Blackbeard is only out for himself. He's a hedonist. He's all about greed and self-indulgence. He's also a interesting mirror reflection of Luffy, which would make sense if Oda's been setting him up as the main villain for the finale. Blackbeard, to me, does feel like a more interesting final villain than Imu does.
Honestly, it's kind of interesting comparing him to Rocks. Because (after only one chapter to be fair), Rocks seems to have a deeper reason for following his plan than Blackbeard does. Rocks seemed to have a sense of purpose that Blackbeard just doesn't portray. Blackbeard just seems to be following Rock's plan...because it's Rock's plan. Blackbeard is honestly coming across as more "shallow" to me than Blackbeard. And I wonder if that's on purpose? Blackbeard LOOKS like a fatter, uglier version of Rocks. And I have to wonder if that's Oda using a visual metaphor to show us that Blackbeard is a "flawed copy" of Rocks. Like a ghost who's only a pale imitation of who they were in their past life, or a clone who's slowly breaking down and dying. I wonder if Oda is showing us that Blackbeard trying to follow this same plan, maybe even being "reincarnated", is actually making him WORSE as he tries to do the same thing, over and over again. A shallow reflection of someone else. A photocopy that gets a little dimmer everytime you make another copy of a copy. A "ghost of the past" who can't move on.
And it does seem like Blackbeard has some form of Rocks still inside him. The "It's probably THEM" line from Chapter 225 hinted at something like this a LONG time ago. But, I don't think it's ONLY Rocks in there. I don't think Rocks has completely taken over Blackbeard's body, or moved his mind into his son's body. For one, Blackbeard doesn't seem to like using swords as much as Rocks did. Now, maybe Rocks is just hiding his Sword Skills as a trump card for later, but I don't think so. Blackbeard used his claws, but most of the time he's just grappling with people, using his Devil Fruit powers. So, I think all hints indicate that Blackbeard is SHARING his body with Rocks, but Rocks isn't completely in control.
Which raises a question of who Blackbeard's potential third "soul" could be? Could Blackbeard also have Davy Jones's soul inside of him? Or the Sea God? Or someone else? Also, if we're counting the "spirits" of the Devil Fruits, I guess Blackbeard could have a total of 7 souls inside of him, all at once, by the end of the story. If he has 3 "ancestors" and 3 "Devil Fruit spirits" in him at the same time, along with his own. If Rocks is in there, I doubt he's tied to one of his existing Devil Fruit. It doesn't seem like Rocks had the Darkness Fruit, so the "ancestors" and "Devil Fruit spirits" are probably separate things. So, we could see Blackbeard jumping from 3 to 7 personalities in a relatively quick time-frame.
As for his crew being weak...that could be on purpose. Blackbeard's crew might not exist to counter the Strawhats. I think it's possible that Blackbeard ends up "eating" his entire crew, just to steal all their Devil Fruit powers for himself. Blackbeard isn't thinking about using his crew as his teammates, they're basically "leftovers" to be saved for alter. That way, Luffy has to fight a guy using more than 10 Devil Fruits all at the same time. Which would make for a cool final fight.
The idea that Blackbeard could end "eating" Imu in order to steal their Devil Fruit is a interesting one. But, personally, I doubt it. Again, I'm halfway expecting Imu to get redeemed. And it would be a little sad if Imu got redeemed only to get "eaten" later. Oda DOES give us some "bad endings" from time to time, like with Ace and Whitebeard. But, most of the time, Oda doesn't like to kill characters off. And if Imu got redeemed, only to die shortly after, it would feel like a failure for Luffy. And at that point? THAT close to the ending? There just might not be enough time for Luffy to get over it and overcome that kind of failure. Also, if Imu does have the Forest God Fruit...I'm not sure if Blackbeard would even want it? I mean...it's a God Fruit, so it'll be tempting. But, I'm not sure if the "Boardgame" powers really fits Blackbeard? I feel like Blackbeard would save his "third slot" for something else. I'm not sure what, but something other than those.
And I do feel like it's possible that Blackbeard's Darkness Fruit is actually secretly the Sea God Fruit, in the same way that Luffy's Gum-Gum Fruit turned out to be the Sun God Fruit. It'd make for another parallel between the two. And his ability to nullify Devil Fruit abilities is pretty similar to the effect water has on Devil Fruit Users. And having the Sea God Fruit would make Blackbeard a pretty suitable villain for a pirate story. It fits pretty well. The Darkness Fruit would just be the darkness of the deep ocean, or of a whirlpool, rather than a black hole. So, if that turns out to be the case, it would make Blackbeard a pretty fitting final villain.
Blackbeard is likely also the person who's the MOST connected to the Will of D, other than maybe Luffy. Or maybe the OTHER Will of D, if there turns out to be more than one type of Will of D, which I wouldn't put past Oda. Two different "D"s, meaning two different things, possibly from two different sources. That would make for a fun reveal and final fight for the "soul" of the world.
Also, as a aside, I doubt Rocks had the Op-Op Fruit. Blackbeard had his chance to steal that Fruit from Law on Weiner Island, and he sounded like he was debating on selling it. (Chapter 1081) If Rocks had the Op-Op Fruit, Blackbeard would almost DEFINITELY want to keep it for himself. That would be a key part of his master plan. So, Rock's Fruit likely is NOT the Op-Op Fruit. It's something else. Maybe the "Sword God" Fruit, which Mihawk or Yoru could currently have? Blackbeard lacking the "Sword God Fruit", might explain why he doesn't seem to have Rock's skill with the sword, even if Rocks is living inside his body. More on that here:
https://worstgen.alwaysdata.net/for...od-forest-god-and-sea-god.69849/#post-6195658
Really, Blackbeard would be a interesting final villain. And it'd feel right, since he's the one who's been set up the longest. The only thing that really gives me pause on that is that I think the True Final Villain will be...someone we haven't even met yet. And the reason for that actually ties in with what Blackbeard's Master Plan actually is.
Because I think Blackbeard's master plan is actually to eventually travel to the "Underworld" and steal something from the very core of the planet. The Chapter 1138 Mural does seem to indicate that there was once something powerful at the core of this world, that the First World tapped into and it backfired on them. I think Blackbeard wants "The World" rather literally, and that he'll end up repeating history trying to do the same thing the First World did, trying to steal something valuable form the World's core. And it's probably going to end up blowing up in his face.
I think the True Final Villain of the story could possibly be something or someone that Blackbeard accidentally awakens and releases back into the World from the "Underworld". Something that's been sealed away since the First World.
And that "something" is probably the "lost race" of Dragons. If you know me, you saw this coming.
https://worstgen.alwaysdata.net/for...message-will-be-the-dragons-are-coming.55673/
The highlights? There's a bunch of dragon statues all over the world, in Alabasta's hot springs, in Fishman Island's palace, in Wano's entrance gates, the holographic "space monster" on Egghead. I think this is hinting that Dragons originally ruled this World until humans invaded the planet and took over, displacing the indigenous people. Humans are the "alien invaders". The Dragons were all imprisoned at the center of the planet and frozen alive. The Dragon's "souls" have been twisted by hatred and revenge, and could be the cause of all the strange diseases popping up around the world from underground sources, as well as the source of the Devil Fruits' "spirits" or "wishes". All of Imu and the World Government's crimes have been the result of trying to keep the Dragons asleep or killing them before they wake up. Imu was trying to stop the Dragons from taking revenge on humanity.
So, I think the True Final Villain of the story could be a army of Kaido-level people that Luffy has to apologize to and find a way to make peace with. Luffy's going to invite the Dragons to his party and try to make up for their unjust imprisonment by sharing a fun party with them, welcoming them back home. That's my bet on how this story is going.
Now, Blackbeard COULD be a common threat that could unite the Humans and the Dragons together. Or not. But, I'll just say, Blackbeard might NOT end up not being the biggest problem during the endgame of the story. Whatever is hiding at the core of the planet most likely is. In whatever form that ends up taking.
So...TLDR? I think it's pretty like that something at the core of the planet will end up being the True Final Villain of the story. Someone or something we haven't even met yet. Blackbeard's my second most likely choice. And Imu's a pretty distant third option. I probably should have just led with that. That might have saved us all a lot of time.