So many great ideas here. Maybe Nolan is secretly stalking these boards to see what people would like the most.
I personally think that the combat should be like Alan Wake, where the use of guns is rare. Maybe the gameplay could be a version of Assassin's Creed-type staying "incognito" to the dreamer. Minus the rooftops. The game has to keep you guessing everytime you "wake up" from a multiplayer match, because it could be part of the other team's plan.
The Animus is the PASIV device in disguise, and what Ubisoft has done with the latest installment, the multiplayer, is a major advancement in innovating multiplayer gameplay. The Inception game should be developed by either: Bioware, Remedy (Alan Wake), Ubisoft, or even DICE (Mirror's Edge).
And if you get stuck in "Limbo" mid-game, then the time difference would be represented by the speed at which you can travel around and build things while the match going on up-above stays in real-time. As you wait for a teammate to save you from Limbo, you will have built yourself your own "place" that you go to every time you die under sedatives, and it's always there.
The graphics don't even need to be that grand, just the stylistic use of sounds and emotive lighting can serve the purpose.
Like, for example, when the you enter a dream, the oscillating sound used when Cobb looks at his watch near the beginning of the film is played. And the different colour palettes used in each level. There should also be plenty of lens effects, like in Amnesia.
Different game modes: Extraction, Entrapment (sending a target to Limbo), Inception, Sabotage (trying to end the other team's plans by waking up and finding where they're sleeping, but you risk being in reality and killing yourself, but you may not be in reality, only a shallower level of dreaming)
Phew, I hope Nolan gets an idea 100x more awesome than anything we can think of!
Abilities: Impersonation (blend into the projections), Forge small objects,