New Level of Units?

[IsaacIvan]

I think part of the reason that the tech tree just sort of ends is because the way the game is, all you'd get is more of the same units, ground, naval, and air. So, would it be possible to make a new class of units that fly over the other ones, and could stack? They could be some sort of spaceship. I think that it could add a whole new level of gameplay. Would it be possible though? And can the game display two units on the same tile?