A multiplayer FPS based around sneaking around and killing other players stealthily. Weapons would be inaccurate over medium-long ranges or would be melee, but would pretty much be one hit kills if you could get somebody with one.
Most of the game would be sneaking around, constantly going "fuck fuck is somebody behind me FUCK I FORGOT TO CHECK ABOVE ME ARE THEY THERE oh God they're not" and then you'd get killed by a guy in front of you because you forgot to check there too.
There'd be little ambient sound or music, but you'd be able to hear things happening pretty much all over the map. So once you get a kill, suddenly you realise that everybody knows where you are because you fired a shot and you go straight back to being terrified.
That sounds like it would be fun to watch other people play.
Especially an incompetent but charismatic LPer
Getting CoD players and the like into the mix, it'll be less stealth and more face-stabbing and trick shots.
However, if we have darkmatch (most-all light sources are off or really dim) as a gamemode, then things can get interesting.
Just as well, if you can allow for other stealth assist items, like an eyepatch, then you can add some flavor into the mix. Why an eyepatch? Cover one eye, and keep it in the dark, at the expense of depth perception; walk through light areas, and in the dark, switch the eye patch to the other eye, or remove entirely, and see better in the dark. Now add Oculus support to this (especially with the eyepatch applied). Now we're having fun.