It could be like how the covenant got started and stuff, so like it would start with just the elites, then you fight against the jackles, then grunts and so on. so you're slowly recruiting all the different races. Then at the end of the game, it's the fight on reach.
But seriously, I'm going to have issues when Halo 4 comes out. I love Halo and I used to play it a LOT, but then I got sucked into Call of Duty and all of my friends pretty much spend 94% of their gaming time on Call of Duty so I do too. Black Ops 2 should be out this Fall, so I'll be heavy into that when Halo 4 comes out. I'll definitely play the single player, but I don't know how much time I'll be able to dedicate to multiplayer (my job is extremely time consuming).
It'd be sweet if playing one game ranked up your multiplayer in the other...LOL Because I definitely won't be able to rank both up. It's going to be either or, and it's going to be Call of Duty. I miss Halo though.
Hard to say since Halo 4 hasn't come out yet, but all I really care about is a solid campaign. I want them to be more intertwined with the books, that's one thing I've been wanting since the books started coming out. Judging from the little information given out about Halo 4, it seems 343i is going to be doing that a lot more than Bungie did, which is awesome.
They've said, if I'm not mistaken that Halo 4,5 and 6 will be a trilogy about the forerunners. Im guessing this will go back to the orinigins of the flood and Chief will either be sent back in time or see the memories of a forunner at some point which depicts their battle with it, kind of like the Protheans in ME3 but with gameplay parts.
I don't think they would make a covenant backstory game, they'd probably leave that to the book. If they did, I'm guessing it would be of the Arbiter, each level your one of the different arbiters up until Thel maybe?