My two cents on the power levels: I recall trying something in a One Piece game ages ago with power levels. I'm not saying we should use it but I'll share my experiences and you can decide.
Basically, the game had three main crews on one island, and to help sort out any potential PvP worries I attempted to implement something from the series. Basically, everyone had a power level descriptor that explained how strong they were relative to the other PCs. These were Captain (the strongest, such as Luffy), Monster (Zoro, Sanji), Medium (Franky, Brooke) and Low (Ussop, Nami). I think I remember coming up with this after taking part in a game in which I felt like four or five players in the crew seemed to want their characters to be Monsters (even though we weren't using the system).
In theory, it was meant to balance out crew strength, but in practice no one wanted their character to be 'Low' as you could imagine, and looking at the current cast maaaaybe Claribel would be Low I guess (but she could just as easily be medium)?
Of course, chances are it'll evolve naturally anyway, just like we can figure the relative strengths of the Straw Hats to each other after considering Enies Lobby and Alabasta, since their opponents had their own ranking system.