Cynical Cat said:
Necrons are a great choice.
One, they're downloaded intelligences in superscience robot bodies. According to 5th edition 40K, some of them are totally around the bend insane and that's by the standards of a bunch of guys who regularly engage in genocidal activities. They are powerful enough to threaten players of Chaos Marine level power and are villainous enough to be clear bad guys even by Chaos standards. They also have hyper tech loot which puts them in the holy trinity of enemies: powerful, clearly evil, and possessing loot worth grabbing.
If they do put Necrons into Black Crusade I forsee melta weapons becoming very popular. Pity about turning all that high-tech loot into slag.