Although I dont deny there are some issues within the current Chaos Codex, i feel there are others that need help more, specifically
DE and Necrons.
I don't want Chaos to become too powerful like the 3.5 'dex as Wraithlord pointed out
I like the idea of having legion specific lists which would mean specific play-styles and expertise whilst not allowing cheesy, dual lash/plague M etc lists. eg. Night lords have a limited number of raptors as troops,
IW's be slow-moving yet possessing a lot of firepower...
and then a list section similar to the current one so you can have your renegades or combination forces...
the only problem with Legion specific lists and watnot is that it could mean difficulty creating uber-cheese lists, but that I wont complain about!

(altho im sure some people will!)
I'd rather not have the tons of stupidly good chaos mutations and what not though, more options hell yeah, but no a 300pts uber
DP or a regenerating vehicle! I also vote for not sharing tons of new goodies with the
SM's as its just not cool and you would soon have people complaining about a lack of contrast!
i would like atleast marks available for summoned Daemons but not as much variety as is in the Chaos Daemon dex, afterall this is a space marine, not daemon army, but they do need some more options in the daemon front...
and finally, lay off the possessed! theyre a good unit! fearless,
str 5 and more! i wont complain! afterall random is whats expected from them... oh yeah a little change such as knowing their special before the battle would help...
just my opinions, say what you like.