Legion/warband specific rules (and/or detachments)
Chaos Lords being equal to chapter masters (Or upgrade-able to similar) And be upgradeable to cult level (i.e Bezerker Lord/Noise Lord/Plaugelord)
Unique characters- (sigh). (Look at Ahiraman compared to Tigerius-80pts more for a model that is worse in virtually every possible way)
Return of the Sorceror lord and the Chaos Lieutenant
Warpsmith should actually compare to a loyalist techmarine (warpsmith is nearly twice the cost for the same stats without ATSKNF or chapter tactics)
Bring back the daemonic gifts from "3.5"
Chaos versions of the 'recent' loyalist tanks(they would steal them or modify what they already have at the very least)
POTMS for land raiders- it is still there in the background
Marks of chaos and cult marines being appropriately priced (mark of slaanesh for example, 2 points for +1 I seriously? Goes double for character marks (15!!!!) especially when you consider that you may want an ap2 melee weapon-which are all unwieldy)
Cultists having infiltrate (they are just worse versions of guardsmen as they are now)
Full sized psychic lores for Tzneetch, Nurgle and Slaanesh (and them being good!)
Daemon princes being T 6. and different stats and points for each different god's Daemon princes(There is a Space marine character that is TOUGHER than them- what the heck!?!)
The "Hellbrute", needs everything that the loyalist dread got in the last codex And to be 5 pts cheaper(chaos have to buy the underslung weapon on the powerfist)
Daemon weapons for each of the chaos gods-not just the one like the current codex
Mutilators and Obliterators should be T5 again
Mark of nurgle should give feel no pain instead of +1 T (T6 bikers is a bit mental)
And decent relics. 2 are restricted to specific marks, 1 requires a melee kill to become a glorified teleport homer. 1 that is a 35pt powersword unless you reach base contact with a specific character- who will be avoiding him like the plague. The other two aren't too bad.