I find currently existing faction quite interesting. At the moment the different factions work, more likely, as a different difficulty levels, when some starting factions being less dangerous and with evolution progress the more powerful factions starts appearing. Like its increasing the complexity of the vanila biter's progression from simple hp/atk increase to actually gaining new abilities.

I see a way to improve the existing Rampant's factions concept by adding some territorial aspect to them.
I thought of the next approach of improving the feel of these being an actual factions in the world, and not just a difficulty levels.
So the concept:
1. Initial map generation spawn, say, 4 base (weakest) factions on their own areas around the starting zone. So those 4 starting faction will be territorially separated at the begining. Like 1 faction on the north, another one on the east, etc.
2. More powerful faction is spawned at the outer circle, further away then starting faction's areas.
3. The expansion mechanisms will eventually mix the distibution of different factions around the starting zone (and player's factory). The aim here is to make this mixing to be finished at the very late game stage.

The aim of the concept is to force player to use different defence approaches on a different lines of defense, responding to different attack types and different damage resistance of factions. Like utilizing more landmines on the north and more flamethrowers at the south. Maybe even make one direction to be way harder then the others intentionally. The "higher" factions will take significantly more time to migrate to reach player's pollution cloud and to start their attacks. But if the player progresses faster - they will reach him faster as well. Another thing here is to increase the difficulty of player's expansion to further placed resources. The vanilla mechanics spawn bigger nests clusters further away from the starting point. The terriorial factions concept could make this difficulty by quality, not by quantity.