Writing. I love the challenge of crossovers, trying to keep all the characters true to canon even in alternate settings.
I also like to indulge in my personal fetish for size-themed stories. Shrinking or giganticized characters interacting with differently scaled lovers.
but... that last battle we JUST had... that was weird... How did you get all your strength back the SAME time I tried to kill you?... that was SO weird...