Question & poll speaks for itself; who do you think will be the main bad guy(s)? I ask this because there isn’t really an obvious antagonist(s) leading into the game.
’Jul Mdama/Storm Covenant would be the obvious choice, but he wasn’t even in the Halo 4 campaign (if my memory proves me right), only Spartan Ops + the Kilo 5 trilogy (and a mentioning in the comics, I believe). And it’s not very original, is it? Just a covenant elite. I don’t know, I guess I feel something is missing.
But in saying that, he does have Dr. Halsey with him now, perhaps adding a bit of spice to his character. Speaking of Dr Halsey, I can’t imagine she would be the primary antagonist. Her fight is against ONI, not the Chief.
The Didact/Forerunners: more-than-likely, but who on earth knows if he’s still alive? I’ve heard so many different arguments, so I’m just going to assume he will return. He has a back-story, and he has a motive, and the Forerunners will come back in some capacity. All that’s lacking is whether he is alive or not.
ONI and Serin Osman: Motive against Dr Halsey, and an evil aura about her. Not to mention ONI are driven by hunger for knowledge and secrets, whatever the cost.
The Flood/ Gravemind? The flood will be in the game, surely. Whoever has read the comics will agree with this. And a Halo 5 promotional cover-art has what looks like flood pods flying over the top of a city (likely Sanghelios). And the Gravemind? Is he still alive?
343 Guilty Spark: Long shot, but he isn’t dead.
Sarah Palmer even? Something about her…
New Colonial Alliance: Probably a favorite, actually. They’re gaining moment in the comics, and will have the Insurrectionists on their side.
Let me know who you think (& why), and if I’ve forgotten anyone. Cheers