I found this trolling around the web and found it rather interesting. It covers all armies and has some great rules involved in it. Big_Willie from DeepSpace calls it a mix of 40k and necromunda. I'm looking forward to play testing it.
The Imperial hiveworld of Icarus Prime, once located in the Agrippinaa system, a mere handful of light years from the Eye of Terror, mysteriously disappeared in the year 850.M41. Ninety-nine years later, sporadic reports of a world matching its atmospheric signature appeared deep within the Eastern Fringe, near Tau space and directly in the path of HiveFleet Leviathan. Forces from all over the Eastern fringe are being dispatched to investigate the ruins of this once mighty hiveworld.
Welcome to Thunderhawk Down:
Generally speaking, the rules for Warhammer 40,000 are used for this game. While the final version will be more complete, for now assume that the normal 40k rules apply with the following additions. Hopefully you will enjoy playing this game as much as we did putting it together. At this early stage, we want constructive criticism on everything. We want additional ideas. We want ways to improve on what we have begun here.