Below is a complete summary of the curent state of rumours. As new information comes to light, this summary will be updated.
Recently, ghost21 posted that he had "made up" his information. Rather than expunging it completely, I will keep it here highlighted in red so that should similar stuff come up later, it can be weighed against this old stuff and people can decide whether it is simply rumour reverb.
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.
Jared Van Kell from Liberium Online supports this split of Codices.
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 date, saying:
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.
theDarkGeneral was supposedly given the Legion split breakdown as:
TastyTaste from Blood of Kittens claims claims "It is not going to be divided into two codexes or anything special. This is not the return 3.5 codex everyone has been wanting this is more a clean up than anything else."
Both ghost21 and Jared Van Kell say Mat Ward is rumoured to be writing the codex. (seriously guys, get over it )
Reds8n contradicts this, saying Ward is tied up with other projects (likely LotR or Hobbit related) and that Phil Kelly will be writing this book. theDarkGeneral has recently agreed with this.
Minor Gods will supposedly make an appearance and all Legions will be included
The minor Gods are named, and will act in a similar way to marks, but have no Daemons.
In March 2012, theDarkGeneral said that the Dex was finished and would be leaving the printers "shortly" for warehousing ahead of distribution "in the next couple of months". He later said that the book should allow players to field Legion-specific armies that have a uniqueness similar to older Dex's whilst simultaneously allowing fexible mixed force builds.
Ghost21 tells us the background has been expanded and improved so that Legions and their worship of the Gods is far less one-dimensional.
theDarkGeneral says the background will include info on “alliances, distrust and full fledged war within the 9 Traitor Legions” with Black Legion referred to as "arrogant"
Each Legion will have something unique to them, but will have unit restrictions to offset their inherent bonuses.
Ghost21 further clarifies by saying World Eaters may be restricted to 1 Havoc squad, but could potentially take more by “trading in” something else.
theDarkGeneral tells us:
Later still he has implied the following Legions could get USRs:
Nurgle units have a rule called "Stoic" meaning they can’t perform Sweeping Advances and will possibly get something akin to True Grit.
Khorne has to “engage in some kind of combat every turn” (not sure what this means ie does shooting count?) or roll on a D6 table and suffer some random penalty
Tzeentch require a “command figure” (likely Sorcerer) or will act based on a random D6 roll
Slaanesh are described only as having “very harsh results”
Recently, theDarkGeneral has claimed that Chaos will likely struggle to combat psykers, but Khornate characters will have ways of nullifying psychic abilities aimed at them.
theDarkGeneral has said there may not be much focus on newer Chaos Space Marine chapters, keeping things squarely on the original 9 Traitor Legions and their splinter factions.
He also said he believed Chaos may get a form of Preferred Enemy, but was unsure which units.
theDarkGeneral says Nurgle Flamers can be made poisoned 3+ or even 2+ (possibly through a special character).
He also thinks Plague Knives will be poisoned 4+ with 6s ignoring Armour Saves. Although he goes on to say it’s likely only unit champions and Chosen/Possessed that get them.
ghost21 claims Legions will have access to Artificer Armour but by a different name
theDarkGeneral says the majority of CSM units will retain Bolter, Bolt Pistol & Close Combat Weapon.
theDarkGeneral claims Daemonic Mounts will be useful, with the Undivided version "worth while" for non-Marked Legions. He also implied that Tzeentchian ones will be fast, saying “Thousand Sons will be VERY pleased with their new found speed.”
theDarkGeneral has implied that there may be an additional heavy weapon upgrade for Chaos units, but was supposedly told it's not a Plasma Cannon
theDarkGeneral has claimed some mark-specific vehicle upgrades reminicent of the 3.5 Codex may appear, such as Poisoned flamers for Nurgle (possibly the same as mentioned above), higher strength/lower AP weapons for Tzeentch, Sonic weaponry for Slaanesh and "brutal blades" for Khorne (a possble reference to the old Destroyer upgrade, maybe?)
theDarkGeneral claims Lash of Submission may get downgraded in line with the Daemons Codex version and that a suite of vehicle-mounted sonic weapons were at some point play-tested for Dreads and Preadators, adding there could also possibly be a new heavier sonic weapon with some extra tank-busting capability.
There was also some suggestion from theDarkGeneral that the Havoc Launcher may be upgraded to two shots.
ghost21 tells us characters will supposedly be very powerful, including the “Red Angel” from this artwork
Black Library author, Aaron Dembski-Bowden has revealed on B&C that considering discussions amongst Black Library authors and the GW IP people, he would be surprised at the Red Angel making an appearance.
Although ghost21 goes on to say he hasn’t seen a named Warsmith character, only a Dark Mechanicus one. Jared Van Kell from Liberium Online contradicts this saying Honsou will make an appearance.
Jared Van Kell says Kharn the Betrayer will become more survivable (likely due to Eternal Warrior and a 4+ Invulnerable save) but goes on to say “his bloodshed knows no bounds”
theDarkGeneral claims we will see the return of Droomrider, Doombreed, and Cypher as well as another 5 new characters. This does not preclude any or all existing special characters being included.
theDarkGeneral also tells us there will be a Slaaneshi Dreadnaught special character. ghost21 also confirms a Dreadnaught Special Character.
theDarkGeneral reports that Ahriman will be better due to new Chaos Psychic powers as well as "some kind of guaranteed psychic ability that can't be stopped via Psychic Hoods, etc"
theDarkGeneral also said that several special characters look to be getting varying levels of Eternal Warrior (1-3), including Abaddon, Kharn, Typhus and two more.
TastyTaste claims there will be no new Special Characters, and that existing Special Characters will unlock appropriate Cult Marines as troops
BramGaunt says he's heard Special characters such as Kharn, Ahriman etc unlock unlimited Cult units as Troop choices, but generic HQ choices with a mark allow one appropriate Cult unit as Troops (similar to Ork Warbosses and Nobz). He also said he'd heard Chaos players will need to nominate one HQ selection as the army general, and that one will determine which Cult units become troops. He didn't seem overly confident of this though, so take with salt.
TastyTaste has said Typhus will upgrade Cultists into Zombies
ghost21 tells us Sorcerers will get something similar to a Psychic Hood, but called something else
ghost21 confirmed what TheDarkGeneral said about Word Bearers getting Chaplains / Dark Apostles
Techmarines will "kind of" be available according to ghost21
theDarkGeneral claims there will be more options for "mid-level" characters possibly able to be taken as 2-for-1 HQ choices, or possibly "as a separate Elite or Fast or Troop choice that can be split off" The example he gave was a possible unit of Dark Apostles that can then split off as squad leaders. This could end up being a sort of Wolf Guard unit, it is not yet clear.
theDarkGeneral claims that HQ choicess will effect the types of units and marks available to your Army, mentioning that "IF 6th Edition returns to Percentages, the Codex will make more sense". He subsequently clarified that HQ selections may change what units "count as" (unclear if this means FOC slot used or counting as scoring etc), wargear upgrades for squads and vehicles, access to Daemons, and possibly affect unit sizes.
theDarkGeneral claims Daemon Princes will be more in line with the Daemon Codex entry but "with a more Chaos Space Marine feel", going on to say they may gain specific benefits for different Legions.
Jared Van Kell says Cult Terminators are likely to return under the proviso that they will only appear in dedicated cult lists. theDarkGeneral claimed there could be some sort of "generic cult" Terminator unit that receives special rules according to the Legion being played. He made sure to reinforce that this was an early playtest, so things could have changed.
theDarkGeneral tells us Chaos Dreadnaughts will no longer act randomly, and are able to take marks, but will not be able to be Venerable. theDarkGeneral also implied we could see Thousand Sons Psyker Dreadnoughts. TastyTaste claims the Chaos Dreadnaught will be removed and replaced with a new unit (essentially a name change)
theDarkGeneral claims Possessed will be improved and supposedly have upgrades either already included or that you can pay for.
theDarkGeneral claims that Chosen will get more upgrade options, but that they will get costly quickly depending on the Legion and wargear you select. Wargear options for them supposedly include Jump Packs, Terminator Armour, all types of Combi-Weapons, Twin Lightning Claws and more.
There is a chance that certain HQ choices can take a Chosen squad as a retinue. He recently contradicted this saying Chosen won't get access to Terminator Armour.
ghost21 tells us there will be a "gladiator themed" World Eater unit. Jared Van Kell from Liberium Online says these are Chrono-gladiators who must keep killing in order to stop a timer in their heads from killing them.
ghost21 also says Emperor's Children will have access to a unit described as "soul shieldsman" which is possibly an upgrade for Veterans.
theDarkGeneral mentions a new unit called Brazen Knights consisting of "Skull Champions" (possibly super Khornate CSM Champions) riding Juggernauts. He has more recently suggested that this could in fact be an Apocalypse formation to be released around the same time.
He has more recently told us that early discussions included the idea of Legion-specific “Daemonic Rider” units, such as Disc mounted Rubrics, Beast mounted Plague Marines, Seeker mounted Noise Marines, and the previously mentioned “Brazen Knights”, but went on to imply development may have stalled because of the question, “What would the other Traitor Legions get?”
According to theDarkGeneral, Possessed will be more viable, losing the "game costing randomness" curretly on offer. Ha also claims they could become like the Ogre Kingdoms Maneaters, picking abilities from a list before the game, and that they could have the (expensive) option for wings. he also suggested a character may allow one or more units of Possessed to be taken as Troops.
theDarkGeneral says Thousand Sons (Rubric Marines) will not get any weapon options, but the new psychic powers can certainly fill that role. He has more recently suggested they may get more unit upgrades, especially for the SGT/Sorcerer.
According to theDarkGeneral, Berserkers could see their Rage rule return to the old Ld test system, where a failure results in Rage as we know it. He also said that 2 ideas for Chain Axes have been floated. One being simply giving them Rending, the other being +1 Strength. It is unclear which, if any of these ideas will make the cut.
theDarkGeneral has made thinly veiled implications on a couple of possible rules. First, Berserker could be looking at a slight point cost drop, and they could have the option for 2 Hand Flamers, plus their Khornate Chain Axes could possibly give a re-roll to wound.
TastyTaste has said all Cult Marines are Elites
God-specific Daemons will likely be included, but ghost21 is only sure of the lesser varieties (Plaguebearers, Bloodletters, Daemonettes & Horrors) and not the greater ones
theDarkGeneral claims Daemons will remain as they are in the Daemons Codex, but with slight points adjustments to reflect the ability to summon them and certain 6th edition changes. He seemingly contradicted this when he said Daemon Princes could be the only "Marked" Daemonic creatures, however what this means is unclear.
theDarkGeneral implies there will be some kind of “Scout unit” which will not be available to World Eaters.
TastyTaste has said Lesser and Greater Demons are gone
TastyTaste has said Chaos Space Marines and Cultists will be the only Troops choices
TastyTaste has said rules for Traitor Guard are in the Codex, but hasn't elaborated on how they appear.