Shanks is final villain, Luffy takes out WG and other yonkou, shanks slips in steal the yonkou ponegylphs whilst they recover from Luffy, meets him at raftel to take Luffy's glory. If we see shanks appear at all at the end of this WCI cover story I'm putting money on this shit
Tbh, I don't think Shanks is "evil" like that. Yes, there's probably a big ulterior motive and yes, he's very shady due to his actions, but tbh I don't think his goal really has to do with One Piece.
If you think about it, One Piece isn't the endgame, it's a tool to be used for the endgame. Because Roger found it and couldn't "use" it and the knowledge he learned to change the world. And Whitebeard said the same thing hundreds of chapters before.
Tbh, I think his "goal" involves getting rid of the WG for "peace" but to an unknown extent or reason that goes beyond the Final War.
It's why I think, yes, he's the final antagonist after the Final War, but it's meant to be an emotional close, rather than an epic one like Blackbeard or Akainu or maybe Im. Maybe there is something far more important with God Valley than we realize.
Maybe that is where the endgame of OP truly happens. Not Laugh Tale. Not New Marineford. Not Mariejois. But God Valley. And with Shanks and Mihawk (there's hidden history there too).