Rumours and semi-confirmations!
Chaos are from the Black Legion
Units:
Chaos Space Marines (obviously)
Chaos Predator
Chaos Dreadnought
Chaos Khorne Bloodcrusher
Chaos Hounds (possibly a wargear item)
Chaos Terminator Lord (based on the very first teaser to the expansion, where people got a Chaos Terminator Lord miniture)
Chaos Sorcerer
Plague Marines
Chaos Devestators
New units for all current races;
Multiplayer:
All four Chaos gods may be represented in multiplayer.
Snow maps.
Singleplayer:
Players continue on from end of current campaign, with all their items and levels.
Level cap raised to 30, new unlockable abilities introduced.
Corruption aspect: Depending on what you do in a mission, your units/chapter may or may not become increasingly corrupted. (such as destroying an Imperial Alter). Wargear drops and stats will change depending on your corruption level. NPC's in missions may or may not assist you depending on your corruption level.
Eliphas may be making a return.