Here's what I'm expecting in the next codex...
Rubrics and Scarabs will go +1 wound (obv)
Warpflamers will push to 12" (obv)
Soulreaper cannon will get go to dmg 2
Sorcerers will likely become our Lieutenants or they will give us a new HQ to fill that role. Someone else suggested they will split the sorcerers in the rube and scarab units like the mistress and repentia which is a possibility. It would also help with Abhor the Witch since rubes and scarabs will no longer count as psycher units.
We will get some kind of rule that there can't be more cultists or goats than true thousand sons and rhinos will no longer transport anything but rubrics
I suspect we will no longer be able to get any better than a 4+ invuln except maybe on HQs. You'll notice they really clamped down hard on that in other codexes.
We MIGHT get a transhuman. Every codex in 9th so far has given out transhuman abilities like they were hotdogs. Maybe this will replace our 3+ and 2+ invulns.
Magnus will get 3 warlord traits and interact with the cults in some way. Perhaps he can know powers in cults and counts as being from all cults etc.
Warptime will be nerfed to Temporal Corridor (SM codex) meaning you can't shoot or charge afterward. I would keep an eye on the Orc codex when it drops because if they get this treatment then the writing is definitely on the wall. I'm hoping we get something like a +2 to charge rolls power to compensate.
I expect a mass overhaul of the psychic trees. We will probably get a lot of identical powers found in other codexes so far and they will take out a lot of gimmicky powers. I like the trees in the other books so far so this is probably one of the things I'm looking forward to the most.
Inferno weapons may get their own special rules other than more AP.
Some sort of doctrine system. Every other army that has come out so far has gotten one. Maybe we could get a psychic power themed doctrine where specific trees receive a bonus depending on the "doctrine" active.
Some sort of army wide bonus. Will probably be like the above^ or smite warp charges staying at 5. Who knows...
Now if we got all of that more or less I would be mostly content I guess. My biggest problem with chaos factions is how badly written their rules are. I'm a new player and almost quit the game when I saw what a mess piecing together their rules from multiple books was.