I'm in the same situation as you and I've also given it thought.
1) Chaos Marauders or/and Beastmen Gors + dakka bitz = classic cultist. I have 20 gors and plan on cutting the sluggas off my orks and pasting it onto the gors to make them more wargear approved.
2) Beasmen head and legs and bits + cadia torso and guns = badass mutated renegades.
E.g.
http://i35.photobucket.com/albums/d194/Gorman2000/Beastmen-attacking.jpg
a bit like this, but moar chaos
3) Get guardsmen, beastmen, etc and greenstuff the bastards.
Just some interesting ideas to start you off.