I think that Chaos will get a pretty comprehensive codex I think, eventually. It wouldn't supprise me if they got it last or nearly last and got the biggest one yet. Allessio Cavatore recently mentioned (on an article that appeared on this site!) how the old Codex was designed to shy away from the Chaos Gods with the understanding that individual codexes, or a codex specifically for the legions, would eventually come out.
I think that sounds fine. A rundown of the existing CSMs shows a codex that really isn't focused much on the "chaos" portion of the CSM, and as such has few of the really crazy cool stuff that Chaos is meant for. They get some great gear, but by and large they are recolored, sometimes slightly respecced, marines. I am looking forward to some really cool marines in the future.
Wait, I hate Marines. Well, at least they'll be different beyond the damn paint...