Here's an idea I've been kicking around for a bit. A codex of unaligned factions that can be taken in any standard force, representing anything from payed mercenary work to mind control (because how else would we have them work for the 'nids without brain parasites?)
There'd be rules for stuff like Kroot warbands, Ork Freebootaz, maybe even a few units of Tarellian mercs or other exotic races who aren't worth a whole codex but are still mentioned in the fluff.
Maybe there could even be rules for stiffing them on the pay, letting you get the units at a discount but giving them a chance to turn on you.
What do the people think? Genius, or Madness (or Sparta?)