We've long hoped for a Gears of War/Halo crossover, and it turns out so has Epic's design director, Cliff 'Cliffy B' Bleszinski. The celebrity developer approached Microsoft over including John 117, aka Master Chief, in Gears of War 3 as an unlockable character, but Microsoft declined Epic's offer, pertaining to Halo's soft M rating, which would be literally torn apart when a downed Chief has his arm ripped off and battered with by a gleeful Locust Drone.
"I'll let you in on a secret," Bleszinski told Complex, "I actually approached Microsoft on getting Master Chief into the game, but those guys are kind of squeamish". It's sad but understandable; Microsoft is very protective over Halo. Bleszinski did admit they might do some unofficial modelling and animation tests and release them online for fans to enjoy, however. There's much more to pore over in the interview; give it a go. [Complex]