TBH, Shanks imo, but not as the final "villain" but final antagonist. Luffy's connection to Shanks is infinitely more emotional than beating someone like Blackbeard, who will more provide an epic finish to EoS versions of our characters.
I think Shanks will survive as a character beyond this point (of defeating Blackbeard and the WG) to lead as the final roadblock for Luffy, in whatever major plot that will occur by this point. Returning the hat is pretty much everything in this manga sans becoming Pirate King. Recently, Shanks has been depicted in a slightly sinister/darker light and I think its for a reason we don't know about yet. Another thing that supports this is Shanks's relationship with Mihawk and Mihawk likely being Zoro's final obstacle as well.
Blackbeard was conceived well over a hundred chapters in to the manga, while Shanks has been present from the beginning. Oda has planned his ending since before the manga began, which leads me to believe the ending more than likely involves Shanks over Blackbeard.
IMO, the order of "villainy" after Kaido/BM (with my own interpretations) seems to be AT THE MOST (in no way shape or form hinted to be happening like this):
- Underworld/Impel Down Level 6 silver medalist related villain. Interestingly enough, the Underworld isn't going anywhere after Kaido and the ID escapees were never addressed. But I do think both will be plot points in the future. I'm not sure if they will explicitly be arc villains though.
- Vegapunk/SSG/Kuma. This is due to my thought that Vegapunk is next. Perhaps the SSG is specifically a "Final War" type deal to counteract Blackbeard/etc as a replacement for the Shichibukai. This will depend on what Oda does post-Wano)
- Elbaf Villains (Loki? Big Mom? Someone new?)- Depends on who is involved in Elbaf and why.
- Marine Admirals/Fleet Admirals/"Zodiac" Vice-Admirals. Funny enough, if you take out Aokiji, there are 11 more Zodiac animals, which aligns with the 11 members BB has and Shanks potentially has. If Luffy gets one more crew member, it works well. Akainu vs. Luffy is a given. Akainu's top Admirals vs. Zoro, Sanji and Jinbe seems like a possibility. At least 2 additional "Zodiac" VA's exist (those with Admiral names but not an Admiral ranking), via Chaton and Momosagi, so its possible the remaining 5 exist in this ranking after losing out to Fujitora/Ryokugyu. Lets just not turn this into a power level discussion, its a fun possibility regardless.
- CP-0. To be honest, I see these being the main battles for the Revolutionary Army. There are 7+ members so far (4 in masks + stussy, Lucci and Kaku) and the Revolutionaries have 6 commanders + Dragon, and they may even get Kuma back as their possible New World Commander in the end. Plus, their goals seem to be at opposite ends. The Revo's want to take down the Tenryuubito, while CP-0 directly protects the Tenryuubito.
- Im, Gorosei, Kong, Kong's subordinates. There are 7 people right there, indicating 4 more people could be working under Kong. We had Doflamingo talk to possibly one of these people after Marineford. Again, not a power level discussion, these people don't all have to be stronger than Admirals, nor do I expect them to be. Is the Monk Gorosei stronger than Fujitora as a Zoro opponent? Maybe? It all depends on where Oda decides to go with this.
- Blackbeard and his 10 Titanic Captains. Not including Aokiji
- Shanks. Has 10 members including Rockstar (if he is a legitimate member), Mihawk would fill that 11th position for Zoro (not joining his crew, just to battle Zoro)
Anyway, this is what I think we can probably expect left at the MOST, and it all depends on what Oda decides to elaborate on and how packed he wants to make these final arcs. Since Oda LOVES hierarchies (typical of Shonen), I can totally see Kong commanding 4 other commanders/WG employees that run the WG, while he himself only submits to the Gorosei/Im. I can totally see the 5 other "Zodiac" Vice-Admirals existing since 2 already sort of prove that others lost to Fuji/Ryo for the Admiral spots. Its very likely Vegapunk runs the SSG in a similar manner, and CP-0 is probably like this too. Reason being is that all 3 of 4 Yonko seem to have rankings like this, the Revolutionaries do as well, Doflamingo's crew did, and more.
Really the extraneous stuff comes from Oda elaborating on the Underworld or Level 6 escapees still, or what Elbaf villain is chosen (whether they are known or not).
I also think Kid will have a major battle with Luffy eventually too (eventually closing out the Supernova plotlines after Bonney and Urouge receive focus), but it depends on how this arc ends and if Kid isn't made to look "lesser" than Luffy after this arc.
We have to remember that Oda will focus on his 10 (likely 11) main characters, so it is likely these people that will face off against the higher up enemies in the end.
Edit: I also think Doflamingo called it the "Throne Wars" for good reason. Wano is only 1 war out of many. One that may pool over to the several wars in the foreseeable future. Another between another major group at Elbaf may occur. It really depends on how Oda wants to take down all of these groups of people, they clearly exist and clearly won't by the end of the story.