I'd like to add another hypothetical gameplay mechanic to what could become a rather samey game:
Although invisible, you still have a reflection.
This could allow you to see your character, which could help for characterisation, and could also help to add a kind of puzzle element to it -- working out what the field of vision is for guards as they pass and ensuring you aren't spotted -- could make for a very unique game indeed.
The whole game wouldn't have to be based around this mechanic, but it would be an interesting one to have to pop up occasionally.
Although invisible, you still have a reflection.
This could allow you to see your character, which could help for characterisation, and could also help to add a kind of puzzle element to it -- working out what the field of vision is for guards as they pass and ensuring you aren't spotted -- could make for a very unique game indeed.
The whole game wouldn't have to be based around this mechanic, but it would be an interesting one to have to pop up occasionally.