While Chaos do need an update, as been mentioned other armies need one a hell of a lot more, DE, DH/WH, crons hell even Tau really need an update more.
Id like to see the legion specific books, but its unlikely, at least until a re write of the existing book, as legion specific ones will want their special chars to be unique to them and as it stands they are in the general codex and it doesn't really work to have two sets of rules for the same guy.
If they were to re write chaos id like to see them make chaos a good separate army, in that I mean don't make them evil marine clones. Don't give them LR variants, or TH/SS Termies and other marine toys, give them their own totally unique stuff, much like the Defiler was a nice new concept back in the day. Possibly instead of giving them legion specific books (which will be a lot of writing and suffer from the classic GW problems of a few of them falling way behind the others), try giving chaos a lot of stuff that effects FoC chart and what units count as troops depending on the Lord, and which deamons become available depend on the HQ choice ect in the next re write.
Either way, don't make chaos just marines with spikes, make them Chaos.
My two cents,