I've been re-reading HBP and at the end, Professor McGonagall calls the remaining official House heads (Sprout and Flitwick) and Slughorn and Hadgrid to the headteacher's office. She says that Slughorn can represent Slytherin, suggesting that he will become head of House in Book 7, but does that also mean Hagrid will become head of Gryffindor now that McGonagall is headmistress?
On other fronts, new teachers will need to be appointed. If McGonagall remains as headmistress, they will need a new Transfiguration teacher and as Snape has gone, they will need a new DADA teacher. Who do you think they will pick? Do you think Lupin could get his old job back? Do you think Tonks could be appointed Transfiguration teacher?
I'm sorry if this has been posted before!