Re classes, the only two that remain that could be different would be Muggle Studies and Care of Magical Creatures. Given the teacher of the former is murdered at the beginning of DH1 and the class forms a minimal part of the books, I doubt there'd be much there. If they were ever to invest in DH2, it could be interesting to see the Carrow twins receiving books - DADA and MS - but again since they tortured students/spread essentially racist anti-muggle sentiment, I doubt there'd be much chance of that happening.
I agree re Umbridge's office. The Troll attack would be a classic. It does feel like we've reached a point where 95% of rooms have been covered since the re-launch, but obviously this soft reboot gives some opportunity. If the next series is POA/GoF focused, there's not loads that comes to mind: Remus's classroom (boggart cupboard would be great), prefect's bathroom, Griffindor Common room with Fat Lady portrait (YES PLEASE!)? If the focus does shift to films 3 and 4, then actually we might see a move away from the castle: Shrieking Shack, Task 2, Task 3, etc. I guess we should also expect Hagrid's Hut to be remade in the coming year or two also.