To expand on the building damage idea:
The buildings wil ltake damage evertime something smashes into them. Some chunks of the buidling will fall off when there's enough damage. Fire can break out if something explodes in the building. If the buildings take enough damages, the collapse. A repair team will come and start rebuilding from the wreckage.
Basically, it's like sticking a real building into the game with real world phyisics.
Repair teams will require time ot repair the buildings. When you leave and come back later, the building will be parly finished. The repair team takes different time to rebuild, depending on the size of the structure.