This is just a "what if" observation, but you wonder if BC had been a thing at the Xbox One's launch, you think the MCC would have just been the Halo 2 remake + keys for CE:A, H3, and H4? Would be interesting to see what that would have been like for people.

It's pretty much what they did with Gears, btw. The Gears 1 remake was also a stealth Marcus Fenix Collection because it also gave you all the 360 Gears titles with the purchase (it was technically limited time offer, but the window was huge), but that also benefitted from being released -after- the BC program was a thing and launched.

And technically Rare Replay, which launched the 360 BC, would have been MCC-like in having all the games ported instead of all the 360 titles being digital BC downloads.