Blackbeard will be the next major villain after Wano. That’s the only thing that makes any sense for the pace and progression of the story. To clarify, by major I mean the main antagonist similar to Kaido in Wano and Doflamingo in Dressrosa.
This means it is possible we’ll have another minor/secondary villain in a quite short arc in between, someone like Caesar Clown in Punk Hazard – maybe Weevil, a giant on Elbaf or so. But this might not even happen at all, maybe we’ll move directly to Blackbeard.
No other character really makes any sense, neither the currently known ones nor new characters:
Akainu will be a major villain, but more of the final arc (unless it’s Imu). Facing the World government for a major struggle before revealing the history and finding Laughtale makes no sense.
Another Admiral doesn’t really fit the location and pace of the story. The Admirals will also be part of the final war, so you can’t “waste” them now. Oda doesn’t want to replace them yet again.
Evil Shanks is just a plain silly theory and doesn’t fit at all to the arc of the whole story. Shanks is meant as a mentor figure, maybe with some shadowy aspects, not an enemy.
Big Mom is down and out. Nothing would be more disappointing for the pace of the story to have her in 3 major arcs, now also after she was already beaten by Kid and Law (even with OP tricks and teamwork). Does Luffy facing Big Mom as a major villain make any sense for an exciting story after Luffy beats Kaido? Just ask yourself this. Does it hype the reader? Not at all.
It wouldn’t really be interesting if Luffy beats her right after she was already defeated and after he beat Kaido. There’re some arguments I see people make that she still needs some closure with Elbaf and Mother Carmel. And that Luffy said he’d beat her. Both are fair, but I personally don’t want her in a third arc in a row as a major (even main) antagonist.
Big Mom may still be in the story, but not in this way. And yeah! Oda is definitely tightening the pace so that’s also why I could see Blackbeard as the next villain. I also think it’s interesting if not Luffy, but two secondary characters beat a Yonko because it cements them as major players going forward. Roger also wasn’t the only legend of his time. There was Whitebeard, there was Shiki, Rocks etc. And I like that this a story about the new generation. Luffy will be PK but he won’t be the only major player.
Any other known character? Who else is even left? There’s barely any super strong people left that would even be a worthy adversary to Luffy at this point. Weevil is no way stronger than Kaido and would also be a super lame main antagonist. Maybe he will be a minor enemy, but not the arc boss.
Aside from Blackbeard this only leaves unknown/new characters. But they become increasingly hard to introduce at this point. How can a super strong character suddenly appear and nobody else has ever mentioned him (unless he is long dead and lived in a closed country, like Oden)? Introducing new characters was fine when first reaching the NW (Caesar Clown), but suddenly having someone on the level of the Yonkous as an opponent for Luffy would seem like a complete asspull. Other mangas do this stuff, but not One Piece. The Yonkous were built up since Water 7!
A minor option would be some sort of evil/boss/king Giant on Elbaf. But we’ve never even heard that there is such a character. Giants are said to be strong, but none of them has ever been mentioned individually to be on Yonkou level. It could still happen, but seems quite unlikely to me.
In conclusion, this only really leaves Blackbeard. After defeating Kaido, the only characters left that seem like proper antagonists for Luffy are either reserved for the final war vs. the World government or they just make no sense from the progression of the story. Everyone else is too weak and unimportant. This really just leaves Blackbeard.
Theory by TooDriven (https://www.reddit.com/user/TooDriven)