Such as, for example, a factory and a farm on the same tile.
But building on top of another building costs 5x the normal production cost compared to building it on an empty tile. Sort of makes sense if you think about it. Consequently, in the late game planets can be built up a lot more than before.
Thoughts?
The issue here is that currently in the late game, after you've filled up your worlds, the only thing left to build is carriers and transport ships. That can get a little monotonous.