[PTeppic]

I would imagine that McGonagall would be head in the short term, as not only do you need a competent teacher but you need someone both the staff and pupils will respect. That in itself might be a problem, since Hogwarts appears to be the only school in the country. Not very good for career prospects, if you think about, and what about teacher training? Anyway, presumably the only alternative will be to either appoint one of the other existing teachers (unlikely?), find a "random" adult witch/wizard who is capable to do it, or, most likely, send someone from the Ministry. Someone other than Umbridge, presumably. At the end of the day, the head doesn't necessarily have to teach, so need only be a figurehead and manager.