Okay, here's a thought for a small plotline waaaaaaaaaaaaaaay ahead in the future for Kazuma.
Y'know how Vizards come around when Hollows infect a Shinigami and whatnot?
So far we've seen all Shinigami at most control their Hollow half.
But it's never implied they cannot be permanently erased.
Just a thought, though. It's something I was considering to give Kazuma more depth and make him majorly badass without resorting to Bankai (which, if he even attains, will be by the end of this game).