Elbaf is Sabaody, except Luffy succeeds in what he sets out to do (save the kids), and In is the one sent away (that or Imu fucks off because Oda is holding off on Imu really doing anything in regards to Luffy (that or more endgame lore is needed).
Lode Star Island has the Ponegylph that’s held by the man marked by flames, and with it, the Straw Hats have the way to Laugh Tale. But since this is Impel Down, we get word that Vivi has been captured, and Luffy and company set out to rescue her…which is also Enies Lobby
As Luffy and company head to Marineford, word gets out about Luffy having the Poneglyphs, and we get all the old characters chasing Luffy for having the key to The One Piece along with some more lore that Luffy is the key to (such as Joy Boy’s promise to the fish men, and how Luffy is keeping it).
Marineford/Enies Lobby 2.0 starts with the Gorosei presenting Vivi to Imu, and we get lore as to why Imu wants Vivi, the Straw Hats and their friends fight the Marines, the Gorosei, and Blackbeard shows up because of course he does, and all hell breaks loose.
Meanwhile, characters talk about Whitebeard’s last speech, leading people to realize Whitebeard was talking about Luffy.
Gorosei and Imu are defeated, the Celestial Dragons lose their power and status, and Luffy does some Gear 5 stuff, and heads to Laugh Tale.
We have our massive showdowns: Luffy vs Blackbeard, and Zoro vs Mihawk. At Laugh Tale, Luffy faces Shanks. And finally, we learn what the poneglyphs say, and the “funny story” that made Roger laugh: my guess is it’s how Joy Boy managed to get The One Piece on the island, outsmarting his enemies to make it happen, which is part of his life story that answers the big questions that have been asked. And it’s all just one big parade of madness that makes Joy Boy something like Bugs Bunny.
And then…the moment. Luffy obtains The One Piece. And declares himself King of The Pirates.
And just as how the first half ended with Luffy talking about how he’s going to get stronger for his friends before training with Rayleigh, Luffy talks about what his real dream is now that he has Pirate King status now, and how he’s going to make it reality.
And we end on Luffy approaching Shanks with The One Piece, the realization hits that Luffy has surpassed his idol, and he returns the hat.
And if Oda wants to do an epilogue, we’ll probably get the Straw Hats basking in the reality of their individual dreams coming true, and then we go back to Luffy having adventures or something.