I was thinking it would be a sweet idea to have full buildings with ruined versions inside.  So if the structure takes damage you can pull the top off revealing a ruined version.  The main problem with this idea seems to be that the ruined version will be smaller than the undamaged one in all dimensions.  Regardless of this effect, I think it'd work well.  I need to buy some supplies first though as I don't have everything I need handy to do this, I was intending to take some clear plastic and paint the back of it blueish black, so it is glossy on the outside, take a Sharpy and draw horizontal lines to separate the floors, then use balsa strips along the edges and periodically along the sides.  Then I was going to make a slightly smaller one that'd fit inside it and is ruined down a bit, perhaps making a solid floor in there so you can stand some infantry.

Just some thoughts, hopefully I'll get this worked on soon.

Would it better to just have a complete intact building, then chop it in half at right places, so when it is destroyed, you can pull the top half off?

You need to choose your cuts carefully so when the building is intact, you cant see the break lines. But I'm sure it's possible.

I thought about it, but figure it'd be kinda shifty.  Anyways, next time I run up to the big town, I'll stop and see about getting the balsa and plastic I'd need to make a building or two to try it out.