The three armies I play just need new books, but Chaos is my first addiction so a few ideas for
FAQ/new book update.
Make Chaos lords cheaper.
More options for Daemon Prince.
Better chart for possessed and/or lower points cost.
Dreads need a better chart and deepstrike/teleport/summoning.
Marks need to do more then just one thing and/or lower cost.
Marks and/or upgrades to summoned Daemons.
Improved sonic weapons, add them to Havoc options.
Larger sonic weapons for dreads/preds/landraiders.
I mean there are lot of things that could be improved, nothing specific but general ideas.
Marks should a couple of things like khorne could +1A, +1WS, furious charge/counterattack.
Nurgle could be +1T, +1w.
Eldar
Warwalkers and Falcons are
FA
Falcons/prisms gain squadron rules
increase range for spirtseer to wraithlord
Orks
burnas for ork boyz
tankbusta bombs for boyz
shoota/bigshoota/rokkit/combis/burna for boyz nob
make ard boyz 0-2 or army wide
lootas are heavy 3 not heavy D3

(also
ap 1, melta, rending ok just kidding but it would be awesome for us orks)
better chart for looted wagons, improved range on guns
fearless bikes and make them 15pts a model.
etc,etc,etc,etc,etc
(whoops more then 5

)