The other senses will and must come into play:
- sounds of water
- sounds of thrumming machinery
- the sense of size of hollow spaces
- heat and chill
- smell of smoke
- smell of animals or acrid monsters
- smell of fresh air or stale, still air
- water vapor in the air
- feel of gravel or sand underfoot
- feel of grass or sludge underfoot
- a sheer cliff that has invisible stairs
- a ravine with invisible bridge across it
- a vast open space with claustrophobic, tiny tunnels throughout
Another thing that could be different and confoundingly cool, is characters are not invisible to each other. So, a fighter slips down a chute to a different level and the party can see him wandering around. He can see them wandering around above, but they can't hear each other as they fumble around trying to find stairs. Same with potential foes. You see an ogre pacing below you. Can he see you? Is this the same level with a ramp, or is there a floor between us? etc.
Any ideas to share?