This is an air door:

The sprite could be better than a bunch of dots, but this will do for a demonstration.

The material "air" acts exactly like any normal color/material, except air doors and air locks can be walked through if the locks on them aren't satisfied. In this case, neither the keys nor the door are used up.

This can lead to some fun situations. In particular, if an air door has non-air locks on it, these locks can block the player. (or objects, but I'll get back to that...) Here's a staircase that cannot be ascended if you have too many different keys:

One other fun idea for now:

While air itself is immune to brown keys, when the locks within are in the vicinity of a brown key, all non-air components of the door get transformed to brown, making combo doors much more exciting!

That's all I have for now, but there are also dusty doors with similar properties. I'd like to make some images for those before explaining them, though.