My first thought was, hey, Great Khans
But it would be pretty much the same as Caesar's Legion - a big band of raiders using ancient conquerors styles. They would have a slightly different culture though, so, there's that.
Aren't they in cahoots with the Followers of the Apocallypse? If so, there is a definite contrast between the two groups, that the hero could use to disrupt either or both groups.
Another possibility is Midwestern Brotherhood of Steel, whom are absolutely definitely hardcore conquerors that I can easily imagine taking over Caesar's Legion areas, especially with NCR weakining them from the other side.
For an anti-slavery good-guy protagonist, the Midwestern BoS are clear-cut bad-guys (even if they aren't clear cut bad guys besides the slavery aspect)
If Caesar's Legion crumbles (which I imagine it will), NCR and MBoS are bound to meet - and their meeting is unlikely to be very happy, considering NCR-BoS relations before, and the general attitude of the MBoS
Not to drag FO3 canon into this too much, and I fucking hated the "trog" concept, but I'm far from opposed to the idea of an organized slavery-based nation rising in The Pitt. Obviously, it can't stay the way depicted there, with a non-society consisting almost entirely of slaves and soldiers - who actually lives there? (who grows the food etc etc etc, same old) But as an antagonistic faction, why not? Especially as they expand, and become more than just a rabble of raiders in 4 buildings
The Chinese could also make a come-back, not only that - but they are allready on the continent. San Francisco seems to have a Chinese majority, as well as a well established rulership, well organized and very well armed (most of them carry G11s and such), sooner or later, an independent Shi city-state in the middle of NCR would become a bit of a sore spot