Another idea which popped into my head.
Only thing is that One piece fans are gonna hate it.
Basically, cursed spirit manipulation in One piece.
"But there are no Cursed spirits!"
There aren't.
But there are devilfruits.
Basically a twisted version of Cursed spirit manipulation, in which the MC goes around collecting DF and their users like Pokemon.
You migjt think, oh, that's too OP.
Yes. So in turn, the "spirits/DF user" he gathers cannot use Haki.
And with Uzumaki, he CAN use it like a blast of Conqueror's Haki, but in turn loses the "DF user/Spirit" and extracts the Devilfruit.
Sounds cool?