I was thinking of doing a scifi warband game using 3d6 (bell-shaped curve etc). It'd use 40k factions as it's jump off point (so you could really easily use the models that you already likely have) but be flexible in it.
The first starting point would be players as a warband of the Holy Galactic Empire (which is not an empire, not really galactic and barely holy), as some low level feudal count. Mass mobilisation of war resources is forbidden, so you can only take a warband of your trusted retainers and maybe a few cannonfodder serfs. The lord himself would be a genetically modified supersoldier in heavy armour and the retainers would be close in power (total of about 2-3 retainers). The serfs would be chaff to be chewed up or to slow down enemies.
Full class system and character development for lord and retainers, but the serfs would be baseline units that don't really get upgraded.
From there, expansion to include Junkers (who scavenged ruins for technology, perfect for orks or similar), Swarmers (evolving insects that can completely change what their strategy is game to game), Exiled Lords (basically Lords with some options removed and others added, for CSM etc) and whatever else. Gameplay would be completely asymmetric, as swarmers constantly reinforced with more and more chaff whilst junkers are focused on "THINGS THAT GO BOOM BIG" and RANDOM TABLES.
Goals and advancement would be asymmetric. As an example, the HGE faction would usually be scavenging for loot or trying to destroy infrastructure whilst Swarmers would just focus on killing anything they could and collecting the genetic material. HGE can advance based on their succeeding and Swarmers the same. So they'd be at conflict but both players could come out of a victory thinking they'd won.
I still think it's a clever idea, just a problem to get to it between doing CompSci at uni, searching for a placement year job and juggling personal stuff.