Slytherin and god help dumbledore because Harry's going to want his blood. Dumb sob is number one on Harry's hit parade me thinks.
Seeing as he's powerful enough to scare Fawkes I would say that Bumbledore is in for a bumpy ride.
Can't see him as a dark lord as he's not interested in taking over the wizarding world but with his wandless abilities that no one yet knows of, it should be most interesting.