i've got an idea, but i think it's way too hard to create.
i was thinking for the theme for hogwarts buildings having those tower shapes and brick/tile colourschemess
forrr the bad guys/voldemorts crew - i was thinking perhaps a snakeyy theme? what you guys think?? maybe using some of that rich black shiny bricks from the ministry? or on the other hand they could have really shabby buildings where they're hiding out instead...?
for Voldemort's army it might be fitting if they had moving buildings. Perhaps not even buildings at all.
i'll explain both:
to represent the movement of Voldemort and the fact that he's hard to trace you could make the buildings of that faction move. So it would sort of pack up to a 'builder' and can be placed somewhere else.
there's one unit which serves as the fortress, it summons heroes and builder type units. These builder types 'transform' in barracks units. Once a unit of dark wizards is finished they spawn around the unit.
both are quite though..