Crossing fandoms over with Homestuck is an addiction. :( Help I've fallen and I can't get up. (I'm mentally doing this with Arrested Development right now, for crying out loud.)

Mostly because choosing classpects is a fun logic and characterization puzzle, and when you've got the classpects chosen you can't help but wonder what a session with these people would be like, and soon and so forth. And I guess the people who make characters into trolls enjoy making designs, assigning powers/blood colours and figuring out troll romance.

I guess any fandom with a well-defined system for sorting characters is fun to crossover with. Harry Potter being the biggest example, I guess.