Baloons, atmospherically buoyant parts, would be a cool addition to the game. I think you could make some very interesting crafts with it, and maybe make a "landing" on gas giants as well.
Buoyant parts could also make for really cool aircrafts. Of course they would only work in an atmosphere, and be only deadweight in space.
This post has been merged into this post and this post has been marked as duplicate.