Questionable Reality - It isn't certain what's real/what you can interact with. This could be good for gameplay-story integration, because there's not enough of that around anymore.
Heights - Spydog suggested a while ago. I still think it could be cool.
Duality - Gameplay based on something (presumably the character) operating in two parts.
Consequences - Your actions affect how the rest of the game plays out.
Loss - That's when you lose things.