Halo as a MMOFPS would be fantastic.
Some years ago I played this MMO-FPS game, Planetside. It's still around today in fact.
The game was painfully mediocre in ALMOST EVERY WAY. Terrain, environments, skills, weapons, world objectives...
But despite the fact that 90% of the game was pretty shitty, it was AMAZING in the one way that's most important. MMO combat.
Being part of a gigantic firefight, 50, 100, 150, 200 people all laying siege onto a base, snipers, people driving vehicles, flying ships, gunners in turrents, ordinary grunts, all of them fellow players, in real WAR FPS showdown. Not some pussy-ass 5 on 5 or 10 on 10 "deathmatch", this was real war. We're talking real starship trooper type stuff here. It was with a doubt more chaotic war-like fun than any FPS game had ever delivered for me.
The idea of a Halo MMO-FPS, with an IP that would actually get TONS of people to sign up, an IP that would require a GOOD budget to actually make the game around the FP combat a good game, sends me into convulsions thinking about how awesome it would be.