I play Elle Bishop, Peter Petrelli and Claire Bennet for Heroes. Emily for Hocus Pocus and Hermione or Harry in Harry Potter. I play Sarah for Labyrinth.
For Het, I play the girls and they're dominant. For Slash, I play my boy as dominant also.
I like having a fight-for-dominance twist to the relationship.
I could also go for a high school line or a historical or time-traveling line where your guy comes from the past or from the future and ends up in my guy or girl's world and they have to learn how to deal or how to get back to their own time.
An interesting idea for my Labyrinth idea is to have Sarah die and be reincarnated years later as a guy.
Cross-overs are fun too! Terminator and Alien? Or Batman and Superman?