They day I see the cheif in a game other than the Halo series, is the day I loose all confidence in 343 Industries to not let the Halo franchise die off completely. Having the Cheif in another series would contadict everything in BOTH series, completly ruining any value the storylines of whatever sieries the Cheif was mixed with, as well as the Halo storyline. Unless you do what Valve did and purposely built the storylines of two games to be able to have crossovers, you CANNOT MAKE A CROSSOVER WORK AND EXPECT TO RETAIN THE PERCENTAGE OF THE CONSUMER INDUSTRY WHO BUY YOUR GAMES FOR THE STORYLINE.
Aside from the fact that crossovers never work, you have to consider that whilst GoW fans wouldn't mind tearing one of the biggest characters in gaming limb from limb, you have to consider that if that happened, the Halo community would be in uproar, and is substantially larger than the GoW community. So I applaud Microsoft for deciding NOT to allow something that would plunge gamers into an armed war in the streets.
Also, you can't put Cheif into GoW, because if Spartans were in that game, even as a small, one pixel cameo, the whole series would change. The Locust would've never been able to get any foothold on the surface, if it were defended by Spartans. ESPECIALLY if one of those Spartans happened to be Sgt. Jhonson. (who actually IS an S-I, by the way).