Well, since you ask, here are some of things I've put in Codex Chaos (and I hope you like them, because it's one of the Codexes I've writen):
Expanded Summoning & Possesion rules
Marks Of Chaos
Chaos Gifts (twenty or so of them, for characters only, restricted in choice by the Mark
the character has)
Chaos vehicle gifts
Chaos Lord Retinues made up of Chaos Space Marine Cult followers (i.e.Berzerkers for your
Khorne Chaos Lord)
Combi-weapons for Terminators instead of Stormbolters
Cultists counting as troops choices if the leader of the army has the right mark (i.e.
Khorne Berzerkers can be taken as troops or elits if the leader has the Mark of Khorne)
Expanded rules for Chaos Space Marine Cult troops
Possessed Space Marine squads (new troop type)
Special characteristics for Daemon Packs/Beasts/etc (i.e. Khorne Daemon Packs ar emade up
of Bloodletters and add +1 to their strength)
Juggernauts (expanded troop entry)
Raptors (new troop type)
Obliterators (new troop type, and my favorite)
Fire frenzy for Dreadnoughts
New Wargear
All the old Special Characters, plus a new one, and Cypher
There is some more too, but that's enough to be going on with.
And no, I can't tell you more now - you'll need to wait until the January when the Codex
and new models are released...
Jervis