From Bols
ghost21 reports that the plan is for 3 Chaos books. Legions, Renegades and Daemons
Daemons are already covered, so the next book will cover the Chaos Legions, with a future Renegades codex incorporating all manner of traitor humans, mutants and Marines-gone-bad.
ghost21 thinks this Legions book will be 1st or 2nd book after 6th edition, which is being released July 2012, putting the release around October-November 2012
Harry supports this, saying:
I do think CSM are the first sixth edition book but not sure if they come just before or just after 6th edition.
theDarkGeneral contradicts this, saying he has heard of 3 separate Legions books with an even split of Legions in each. He claims 2 are planned for release in 2012 and the third for early 2013. He further claims a “Lost and the Damned” style book and a “largely Daemon-based” campaign book are in the works for after that.