I have had that thought a million times but have no single great idea on how to implement it and you'd need an actual DM who knows what they're doing

Ok so hear me out, you have a simple homebrew class system based on 5e(maybe pathfinder) have very rare devil fruits that count as subclasses, like Ur a swordsman who eats the chopchop fruit: a chopchop swordsman then you have characters be made based on what roles you need in the crew