If you could choose the next 40K army...

[Lovejoy]

There’s only one faction we need, and it’s time for them to come back. You know who.

[me_in_japan]

@AGP - would that give them access to Daemon Engines and whatnot? Defilers and other such heretical know-how?

[AndrewGPaul]

Well, they'd all share the Chaos keyword, so they could be in the same detachment. Remember, it's the faction keywords that are important, not which book they're in.

I'm sure I've missed a nuance, but the basic idea is sound, I think. All you need is some slight tweaks to what's already there, not a whole new list.