Greenskins! Tough troops, bad aim. What counts-as idea to slap onto their rules?
Lost and the Damned! Traitors, heretics, mutants, cultists and more! This is an idea I almost did, but since I was just finishing up a regular Ork army, I decided to try my traitors using the IG book instead.
I'm not going to go into too much detail, since most of the unit choices are just Orks with different gear. Appropriately armed mutants/techno-barbarians can serve as Orks, with either tough mutants or renegade Ogryn as Nobz, while cultists/human draftees can serve as Gretchin. Ghazghkull or a Megaboss would make for a good terminator-armored cult leader, and Weirdboys would make obvious Sorcerors. Add in some stalk-tanks or other monstrosities as Killer Kans/Dreads and various civilian vehicles converted for cult/military use and you're ready to overthrow the planetary governor.