Now what faction is missing in 40k? I think the 40k version of the rat-people plague that is the Skaven would be the awesomest addition right now. If they already exist, it sadly slipped by me.
So this is my version of the Starskaven, who breed in the lightless burrowworlds, which make macropoles look like pastorial villages. They attack from their space-U-boats, passing by undetectable as they shift through the unknown depths of the Netherwarp (tm), only to emerge and shockswarm planets to devour and rob them of all their ressources within days or even hours. Common weaponry include viral and bacterial bioweapons as well as corrosive gases which melt away resistance before it even arises, just to make the remains even easier to digest. Heavily modified by crude cybernetics the Starskaven throw their kind into battle like we shove coal into a furnace. With endless disposable masses, they swamp their victims, all for the glory of their plaguemothers.
Behold the brood.