With me, I tend to think of crossovers for the characters rather than the story. And so far, I have crossovers for Movie!Kazuo.
I can actually see him and Yoichi Hiruma from Eyeshield 21 being bros. Kazuo-Yoichi broforce! People should run for the hills. (Even though Hiruma is a good guy deep down, Kazuo would surely respect him, and would probably just pretend to be friends with him for the weapons : P )
I also think he'd be rather happy in the Hellraiser universe, especially with the bleeding walls and the full on sadomasochistic torture that the Cenobites inflict on the humans (he'd watch, I think that the Cenobites would spare him).
I had more ideas...uuuuuuuuuuuuh. Kazuo could be a good serial killer in Baccano! I wouldn't feel bad if he kills any of my favorite characters, since they've tasted the elixir of life. And since they're in the same universe, Durarara.