Well, here's the thing. I could see two different paths that Elbaf could take.
If this is a shorter arc, we might just have Prince Loki be our "antagonist". Someone smaller scale that isn't much of a threat, as a sort of breather between bigger threats. Someone that might even get turned into a friend by the end of a short story. This arc could be a way to give the Strawhats a bit of a break before the Final War breaks out.
If Elbaf turns out to be a longer arc, I think we're going to end up bringing in some sort of exterior threat that's going to burn Elbaf to the ground.
Because if Elbaf turns into a longer arc, I don't think it'll be a traditional "Kingdom Arc" with it's own self-contained villain and problems. Because, to me, that would actually be a step backward. We're in the Final Saga. Supposedly, Oda probably wants to keep escalating the conflict until we reach World Shattering levels for the conclusion, perhaps literally. A "Kingdom Arc" with a villain who's just making trouble in this one country would be a bit of a step backward after we just got done fighting the Gorosei, the rulers of the entire world. So, we would have to find a villain who isn't Final Boss material, but who would still be a step up from the Gorosei.
Garling is possible. He did just get a promotion and probably wants to prove himself. But, I'm not sure if the Gorosei showing up AGAIN on Elbaf would really be the escalation that we need in this story. Even with someone more bloodthirsty like Garling now heading up the Gorosei.
Akainu would work pretty well as the person to burn down the Giant's World Tree. But, I have a feeling that Akainu's going to have his hands full with Dragon and the Revoluationaries. And nevermind about whatever Blackbeard's cooking up. I think we're going to see a pretty big fight as a B-plot during Elbaf between all these guys trying to claim Mariejois. So, that's a good chunk of our villain candidates tied up with each other, at the moment.
Shanks isn't a impossible choice. We just saw some foreshadowing that may point to that this last chapter. It'd give us a more personal antagonist for this arc. So...maybe on Shanks.
Alternately, Imu themselves could show up here. Finally getting to see Imu's "Yokai" form would be a step up. And Imu revealing themselves would be a step up from the Gorosei.
Or if Elbaf turns into a longer arc, I could see the antagonist being...everyone, because I could see Elbaf being the site of the Final War. I know a lot of people think the Final War has to come AFTER we find the One Piece on Laugh Tale, in order to punish the World Government for their crimes. But, everything that Oda has said in interviews seems to point to the Final War happening BEFORE we get to Laugh Tale. So, maybe EVERYONE starts fighting on Elbaf? Where better to start Ragnarok than the Norse inspired island?
But, personally, I'm still expecting Elbaf to be on the shorter side. I think Oda has other places he wants to go more.
And...look...I know everyone's kind of excited for Elbaf, because a lot of people are expecting it to be based on Asgard, and be full of Norse Mythology references.
But...Oda doesn't really do that with his islands. I mean, Alabasta was based more on Egypt, the country. Not Duat, the Egyptian afterlife. We don't have places that are based on Olympus to meet Greek Gods, we have places that are based on Italy or the Philippines. Oda usually makes the countries in One Piece based on actual real world locations. With just a few mythology references sprinkled in here and there, usually in the form of Devil Fruits, or in other ways in typically broad forms.
Chances are, Elbaf will be closer to Scandinavia than Asgard. We've already seen that with the Legos since their headquarters are in Denmark. And I have no issue with Scandinavia! It's probably a lovely place to visit. But, it's not as interesting as some people are probably imagining that it could be. Most of what we've seen of Elbaf so far is just...huts. Not terribly interesting huts. And big trees. I just don't think it's going to be that interesting to visit, once the novelty of seeing tiny Strawhats interacting with BIG THINGS wears off.
I feel like the hype for Elbaf got a little out of hand. Which is honestly fair, since people have been waiting for Elbaf for twenty years now. But, I just don't know if it'll be as interesting to ACTUALLY visit as it was to IMAGINE visiting. If that makes any sense. I don't know if Elbaf alone has enough going on to justify a 100 chapter arc like some people imagine it could be. At least not as part of it's own self-contained Kingdom Arc.
So, personally, I'm betting on this being a shorter arc with Prince Loki as our mostly comic relief antagonist. To contrast what's going on in the rest of the world. Mostly, the War for Mariejois, between Dragon, the Marines, the God's Knights, and Blackbeard.