I want a cross between Sion, Nihilus and Sidious. No, they're not going to do a threesome for you, you damned perv. xp
Okay, a guy who has mastered the Force so well, he can change his appearance. He often appears before you and (mis)guides you and leads you into his trap. He is extremely powerful, has a broken appearance (by default), like Sion, wears spooky stuff like Nihilus (in movies and such), and has a mind like Sidious. It'd be interesting to have an end scene where you learn that one of your team members is the Sith Lord and you have to square off against whom you think it is. Man, I'm a genius!