If they did it would be the only shooter ever to have the best of both worlds. Halo 5 has to go big. MS is going to throw money at this game like none other. They want this game to push the Xbox One. I think MS would sell a shit load more consoles right now if 343 randomly dropped a Halo bomb on everyone before November 22. Something like a Starry Night trailer and all this Halo is dead stuff would get hit by a hype train.
But fundamentally, you're selling two games as one, and I don't think that's a good idea. Better to have two completely different teams, two completely different sustains, and two different marketing pushes to explain how this appeals to Segment B who might not touch a game like A, Halo or no Halo. I've certainly said on here way too many times how I'd love a larger, Battlefront/Battlefield-styled Halo, even as Invasion and Dominion sorta kinda' scratched that itch, but I think I've seen enough to conclude it's not feasible in the 16 player cap, just as I really like that 16 player cap for other reasons.