This is not a theory on what will happen in the final act, with the One Piece and Laugh Tale and all that. It’s a theory on what will happen to each character after the climax, like where they’ll end up after the story ends.
Luffy: All my theories about the Straw Hats crew rely on Luffy dying in the end. I think One Piece mostly tells his story and that of the Straw Hats and if he doesn’t die they all stay together and their story doesn’t really end there. There have also been many hints throughout the series, as he’s been shortening his lifespan repeatedly.
Zoro: I think Zoro will take up a sort of Rayleigh role. He’ll become the world’s strongest swordsman and after Luffy dies he will continue his journey alone. He will obviously still remain in contact with the other Straw Hats but he wouldn’t follow anyone other than Luffy and he isn’t necessarily cut out to be a captain.
Usopp: “The weak don’t get to choose how they die.” Well, God Usopp won’t be weak by the end of the story and he will have chosen his end all the way back in Chapter 119. (See picture below). I don’t think he will die during the story though. Maybe he’ll replace Heracles and look after the Boin Archipelago. That way his prophecy would come true and it wouldn’t be sad.
Nami and Sanji: While Oda doesn’t write much romance, I do think Sanji and Nami will end up together. Sanji will have found the All Blue by the end of One Piece but Nami’s dream is a bit more complicated. She wants to draw a map of the entire world. This means she’ll have to travel the world and I think Sanji will be right by her side. Maybe they’ll even start a family.
Brook: He’s the most obvious one I think. He’ll reunite with Laboon and travel with him.
Chopper: His dream is to cure all diseases, which is pretty impossible but he’ll do the best he can and open up a hospital and continue his researches. I could see him ending up with Milky (the reindeer mink).
Franky: Franky will go back to Water 7 and work there as a shipwright. He’ll be known, just like his father Tom, as the man who built the Pirate King’s ship.
Robin: She’s the one I really don’t know, because just like Sanji she will have achieved her dream by the end of One Piece. Maybe she’ll just join one of the other Straw Hats. I could see her and Zoro becoming roomies (or more).
Jinbe: Fishman Island will need all the protection it can get if Luffy falls, and who better than the Knight of the Sea? Whether the island is still on the bottom of the ocean or on the surface Jinbe will be there.
*Theory by YannickSD