An OP MMO or a fully explorable single-player OP RPG might be better than the manga if done right. I just hope they don't introduce some crazy lore idea, like everyone in the future has greenblood or artificial devil fruits of all types are more common to explain why multiple players can share the same devil fruit.
Imagine an MMO, where if you died enough times, you permanently died, and your Fruit got reset? So, people would be constantly fighting to try to kill the Pirates with the strongest Fruits?
It's an unlimited endgame for an MMO, no matter what, people would always be fighting to win.
So much you could do. Have the MMO introduce one part of the world at a time, starting in one sea, probably the East Blue. Or, make it so people can choose which sea they start in: the East Blue is easiest, the others have different difficulty levels that get harder.
Subquests where you infiltrate the Marines.
Imagine a Marines vs Pirates MMO????