You're right, Halo centered would be good. And you're right, not all members may have those games. And you're right that gears has no theatre mode. Fortunately I have a solution for all three.
1. We film just parts of the show on other games. Not all of it, just parts of it. The "HUB", you might call it, would be Halo. Probably Reach, because I only have one copy of Halo 4.
2. If someone does not have one of those games, they either don't have to be in those parts, or, if they want to be, I have... "Minions"... who help me film. They're not the best, but they will do...
3. Gears doesn't have theatre mode, but it does have spectator mode, which will allow us to film. However it could also make it much harder in some places.