Soooo - first we have some information from reddit, seemingly taken from the december WD:
'Nine is the magic number this month, as within this 136-page softback supplement to Codex Chaos Space Marines are army special rules, exclusive Detachments, Warlord Traits, Tactical Objectives and Chaos Artefacts for each of the Traitor Legions - the nine legions who followed their Primarchs into treachery and heresy as they turned from the Emperor's light. (avail 10 Dec)
Loyal sons and daughters of the Imperium take heart, as whilst the Ruinous Powers may seek to be in ascendance as the year wanes, the God-Emperor's most righteous agents stalk the stars ready to enact his will.
Codex: Imperial Agents contains nine subfactions of the Imperial war-machine, from agents of the Inquisition to Wyrdvane Psykers of the Adeptus Astra Telepathica to reinforce your armies of the Imperium. After all, Deathwatch Kill Teams and Imperial Assassins are just as likely to be found working alongside regiments of the Astra MIlitarum or Space Marine strike forces as they are working by themselves, giving the Armies of the Imperium increased tactical choice and scope to conquer any foe.
Within this volume you'll find background and rules for using nine smaller factions alongside your existing armies of the Imperium, from the Grey Knight Terminator squads to the BATTLE SISTERS OF THE ADEPTA SORORITAS."
So, take it with a bit of salt, but I just want to add my two cents:
- I heard that both renegades and traitor legions would get love and rules this year, and I heard there will be two books for chaos this year (but no Codex). Now we got Traitors Hate already, so the legions book would make sense with what I know.
- Physical rules for assassins, sororitas silentium and custodes make sense too. Even more if they can add a teaser for the sisters of battles^^