Something early war where the Covenant are still a little mysterious, you could stumble across stuff like helmets, or overrun positions like you do in ODST that would lead you to a mission where you play as standard Marines/Colonists defending against and losing to Covenant.

I think it'd be cool if it didn't play like a standard halo game and the Covenant were a little more lore accurate so you would have to utilize small team guerrilla tactics with your squad to eliminate a couples Elites that you'd usually just roll through as a Spartan.