I think there's a lot more you can do with Halo. The Banished are actually a neat concept that I think is tapping into some of the potential. Covenant remnants are a logical path to take because of course those kinds of things would spring up after the collapse of a gigantic hyper-zealous empire. There's also ONI you could deal with, but I think they'd avoid that as I don't think they want Master Chief to be mowing down humans. And of course there's the Flood, who even though they were "defeated" at the end of Halo 3, can pretty much never be totally eradicated as outright mentioned by Gravemind himself.
Honestly, I think Halo 4 had a lot of the right concepts but they were just, again, poorly executed. Bungie set up Chief drifting to a Forerunner world in Halo 3's Legendary ending, so the Forerunners pretty much had to be involved in some way. There's a theory that Mendicant Bias intentionally sent Chief to Requiem knowing he'd awaken the Didact so as to prove he'd redeemed himself after helping the Flood genocide the Forerunners, but it's (AFAIK) never explicitly stated. Chief fighting a bunch of robotic Forerunner minions is fine, although the whole Promethean faction was laughably undercooked in Halo 4.
I think one of Halo 4's biggest failings is that at many points during the campaign, all subtlety and logic was just totally thrown out the window in order to deliver by-the-numbers action movie schlock. Didact was cool in the Halo 3 terminals and expanded universe, but in Halo 4 he's just a crazy supervillain who wants to destroy the earth for some reason - oh and be sure to buy the books to find out why he went insane (seriously, they didn't even have the god damn courtesy to mention his mind-rape at the hands of Gravemind in Halo 4's terminals!) Librarian was kind of a deep character, but now she's just an exposition dump machine in what was probably the worst cutscene in the entire game.
Despite all that though, I've warmed up to Halo 4 over time. Don't get me wrong, it's highly, HIGHLY flawed, but my real ire is directed towards Halo 5, which I felt took all the potential Halo 4 built up and flushed it down the toilet in favor of this out-of-the-blue nonsensical evil Cortana plot and this random guy named Locke kinda-sorta hunting Chief for extremely flimsy reasons which really resulted in fucking nothing at the end. I could probably write a dissertation on all the reasons I hate Halo 5, but I'll leave it at that for now.