Might it be possible to create a brick that exerts its own gravity (for space stations, so you can walk normal), or, if you manage to create the atmosphere thing, bricks that create a small area around them that allows your guy to, well, not die?
Also, I know that making the planets move would be impossible (and impractical), but is it possible to make a small object circle around the map (such as a meteor or comet)?
Thirdly, as a secret, how about a micro black hole hidden in some corner of the map? When you get near it, you will start to be pulled towards it, faster and faster as you get nearer until you hit it, killing the player instantly. This would probably require that you'd be able to create spheres of gravity around an object, all pulling you towards its center (or something like that).
I have absolutely no experience with this sort of thing, but I'm willing to guess that none of this is possible.