oooh - good question. Jack has so many sides and personas to him it's easy to figure out a way to have him work with nearly anyone. And with his history and longevity it's possible to slip him nearly into any fandom. Honestly though, other than NCIS, I have yet to really move from writing the crossover to writing in that fandom itself, so maybe it's Jack that helps me come up with the ideas for the stories.

*babbles* I really have no idea. Maybe it's just that Jack's broken in so many ways, yet strong in so many others, he's fascinating to write for and stretching by having him face situations and people we haven't seen him face is a way of testing his boundries