If Blizzard were to rehash 1 dungeon or raid, updated for level 90, which one would you want to be brought back? They rehashed onyxia for level 80, Naxx for level 80, but what dungeon / raid would you want brought to level 90?
Personally for me, I would like to go back into Karazhan at its full might and glory at level 90. Would be awesome, with updated gear and models and such. Plus there are a ton of bosses in that raid, so lots to do, and many people would be familiar with some of the mechanics of the fights, (of course some would change and be updated), so it possibly would be better for casual groups to progress through.