Projected textures would be a great add-on indeed. We could fake light coming through grates, protected lamps, windows maybe even animated shadows coming from ventilators or swinging trees.

This could add alot to every level.

It could be a flat projection or bended similar to the decal-projection and it could be a cubic projection. The last method could also be used to create static shadows via shader. I saw this often in modern games.

Models, Textures and Games from Dexsoft