I had originally intended to use this in a fan fic but it went horrible so I guess I'll just write about it here
A fusion a technique in which you can fuse two people (and animals) together (a devil fruit ability that has no advantage for the user because he can only fuse other people)
first pair luffy and Zoro
appearance: you can comment on your own idea because i don't know what to think of
benefit: a great swordsman with a reach no other man can counter. increase in raw strength.
disadvantages: (yes there are disadvantages) An even worse sense in direction, so bad they try to find short-cuts (long-cuts) to face an enemy right next to them
attacks: using luffy's ability's he starts out like luffy would a gomu gomu rifle but with the sword in his hands to creat kind of a miniture three thousand worlds. (you can give it a name)
I have to go I'll post more latter please comment