Well .. here is what I would do anyway.
Heretics - 'play dead' Heretics should be able to play dead. If your fast enough to micro you should be able to drop a heretics life down to 1 hp and have him fall over. When the threats gone they can get back up. That would atleast give them a way to get away from anything like a scout rush. Everyone else has a way around it. Why not Chaos? Orks you just can't, SM have there own scouts and well, Eldar have bone singers who can teleport. Heretics have forced labour. Wopdee doo ... Of course you could only do this if you have hp.
Cultists - Faster Reinforce and Beter Mele ability. Give them some decent mele ability. They are great with GL's but there crap at mele, this is a chaos race, mega h2h guys come on. Give them more damage atleast then you can use them to tie up other squads and perhaps do some damage. Nothing HUGE just enough to make them slightly more versatile.
Horrors - Give them the ability to reinforce. They are good as they are, with there ability to deep strike giving them even a slow reinforce means you can recycle them rather than creating an entire new squad. There already good as they are. For there cost they have a great deal of ability. Even at %50 to hit, thats 6 of them. A fully upgraded squad of marines can carry 4 plasma rifles that do less damage and there more expensive to get that far.
Defilers - Change the BC to 'player fire' only. Where as they are currently stupidly innacurate. Adding some micro would mean they became useful again as you wouldn't have to worry. Drop the power cost by about 50 - 100 and they would be excellent units. The reaper auto cannon is more than enough to take down a great deal of vehicles and in hand to hand there near unstoppable. Slamming a dreadnaught with the reaper auto first before engaging in H2H is certain doom for the Dreadnaught even at the defilers current hp.
Predators - These things are strong. Yes. But there still to expensive and there anti infantry. Thats an expensive unit for anti infantry. What they NEED to do is make the twin las cannons available at level 2. The primary cannon could use a bit of a damage upgrade, even if it requires a bit of a cost increase to gain. It would make the pred viable as a level 2 vehicle, wich is STILL expensive. And give chaos more option to either use it as anti infantry or anti vehicle.
Obliterators - Lets be serious, these guys are great but there not what they need to be. They need to be stronger. Face it, they should be next to unstoppable, people should quiver and retreat when oblits deep strike. Terminators are scaryier at the moment. Give them more damage and make them much more worth the effort. In the beta they wer unbelievable. What happened?
Blood Thirster - Needs some more HP. Face it, everyone else gets huge benefits. Give the thirster more damage. Make him soemthing to fear. The avatar gives pop cap, the landraiders are well, landraiders so what. And the squigoth, well. Damn! But the thirster really needs some pizaz. Hes good but hes just not strong enough. Especially in h2h. He looses life too, I mean, thats ok I don't mind the fact hes slowly dying. Atleast let him drain a decent amount or something when he kills infantry!
I don't think I'm asking for much - it addresses 'most' of the current issues. What do you guys think?
:EDIT: - these are ideas, I'm not saying everything in each thing should be implimented, there just a compilation of random thoughts.