I think they'll stay with actors and actresses from the British Isles or Australia. I'm thinking Helen Mirren or Dame Judi Dench would be interesting casting. I would have suggested Miranda Richardson, but I think she's going to do a bang up Rita Skeeter!!
I bet you're right, given previous casting decisions. Judi Dench would probably do an excellent job as Umbridge. I think Helen Mirren is a wonderful actress, but I can't see her in that role. Umbridge needs to be played by someone who is short and pudgy. Maybe Imelda Staunton or Vanessa Redgrave?