One thing I might try once I beat GR again is play the game through on elite again, with no TI and nothing else on screen.
Multi-player is always intense, like the one individual said, try the tactical gamer group. If your too good for public servers, start going "pro", build on your already well skill set by escalating yourself to the next level.
Modding would be pretty sweet too, it could be like a side project, make a map, add new weapons, etc.
With everygame someone is going to get bored, it's a given. I haven't experienced it with GR yet, and probably wont for a long long time. But everyone's diff.