Whether it would be in novels or games, I think we'll know of more species before Halo is over.
We know that the Flood originated from somewhere outside of the Milky Way galaxy and we know of the crashed unknown ship we see in one of the terminals in Halo: Combat Evolved Anniversary (I think it was in that anyways).
There must be more intelligent life out there, in the Halo Universe, other than the ones we have seen and read about so far.
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The galaxy alone should have thousands of different sentient species. Cryptum stated that the ark had over trillions of different species. I wonder why we haven't heard from them yet :( Bungie had great plans for Halo 2 and the Halo MMO was supposed to feature many new sentient species. I hope 343 explores more alien cultures including the Covenant. It's funny that we now know more about the once mysterious Forerunner's culture, then we do about the grunts, hunters, drones and other covie species(excluding the Sangheili).