One Piece Unlimited World is kinda "what if" game with anime story.
Hmm....a hypothetical canon might be interesting. I'll have to look more into it.
Basically, what I really want out of a game based on a manga property like this is to be able to interact with the world myself, exactly like the characters in the manga do. Whether it's as an OC avatar or known character, I'd prefer to play my own path rather than the set path of the manga storyline. Gosh, imagine being able to create your own unique pirate crew, name it, customize powers and skills, build your bounty by the actions you take, and meet characters from the main canon along the way. I would eat that up!
It's funny, since I usually prefer manga over anime due to filler arcs, with the pointless OCs and general auto-conclusiveness of it all...but I find that sort of thing would be ideal for a video game.