I think the best way to do a One Piece version of FighterZ would be to not do it in a purely 3v3, but instead make it a variable that can make you stronger or weaker. Say you choose Luffy w/ Zoro and Sanji - each of them are good but maybe lack certain buffs that make them more powerful.

Luffy with Zoro and Sanji gets acess to Gear 2, but Luffy solo can break out Gear 5 for example. To offset having more available characters, the solo Luffy has higher HP - maybe even multiple HP bars if they enter solo.

For the team fighters, the more people they lose the stronger the last one gets by accessing more of their moveset. But not getting the additional HP bars that the solo player would get.

I know that there a hundred issues with this, but I think this would really nail the feel of a One Piece battle.