Epic Games characters have appeared into other games before - Lost Planet 2, for example. The studio's willing to consider any reasonable tie-in with its own games, too, so you're not the first to wonder how awesome it'd be to have a Master Chief skin in Gears of War.
The problem, as Epic Games design director Cliff Bleszinski told Complex, is one of tone. "Gears is like Mortal Kombat and Halo is like Street Fighter," he said, drawing a distinction between Halo's "soft M" and the rougher edges of Gears.
If we were to get Master Chief, we'd get his model, put him in a COG suit, make it blow up, and do some test animation work. We'd have to see what his head looks like being blown to smithereens or how his arm would look getting sawed off.
Bleszinski told Complex he approached Microsoft about such a cross-over, a natural in the minds of both series' fans. "I'll let you in on a secret. I actually approached Microsoft on getting Master Chief into the game, but those guys are kind of squeamish," he said.
Evidently it all went nowhere. We're left to guess the reasons but, yeah, Chief getting mutilated might be a bit too much for his handlers.