My personnal picks
I think both of them are very violent characters and should be in shows with more violence or blood.I would see Kenpachi fit into Violence Jack
pretty well and I would think Natsu being the least violent could fit into Naruto or FMA,but I would like to see him actually burn poeple to death or punching someone with fire and instead of having them fly very far like in Fairy Tail (whatever sense that makes if I hit you with a baseball bat the mass of the bat actually decreases so why would you be hit harder
Another reason why I am bringing that up is my dislike for Fairy Tail and Bleach ,I think they are good characters but there potential could be exploited in another show.
So wich character do you want to see in another show?