Re: Halo 5 speculation (spoilers for Halo 4)
The Prometheans didn't do much for me. They really had no personality. At least not compared to Brutes and Elites. The design wasn't bad though.
But yeah, some aliens with no relation to the Covenant or the Forerunners would be nice. I don't see it as farfetched canon wise, since logically, if the Covenant and Humans didn't run into each other until centuries after they achieved interstellar travel, and many of the Halos are still undiscovered, then there is a lot of unexplored space.