Fai said:
The more I think about it, the more I think that Kenpachi's sword ability will be healing(which would be the opposite of his likes and complement the whole master-student vibe with Unohana) and he will heal Unohana. This is ESPECIALLY likely because he heard his sword the exact moment he actually cared for someone else living on.
I like that idea.
Also it will be totally on his like. When he crush an opponent he had fun fighting against he will be able to heal him to continue having fun.
The "fun" stopping is pretty much a trauma to him, that's why he limited himself to begin with.