Chaos
-mark specific sub races
-Tzeentch
-Khorne
-Slaanesh
-Nurgle
Possibly all of them have a unique playstyle, for example, Tzeentch will rely more on psychic attacks and ranged combat whilst Khorne is has insane melee damage. Or Nurgle with stinky stuff :P and Slaaneshi followers be resistant to morale. (just ideas)
Space Marine
-divided into sub races as well
-1st company
Contains (expensive) elite units and vehicles, special heroes with special powers (also expensive)
-Standard
Usual SM units, cheaper alternative
Eldar
-Give them more Psychic attacks/power (they are the most advanced in Psychic warfare...)
-Scorpion Tank
-Stronger and bigger Avatar of Khaine
-jetbikes (like DE reaver only eldar :P)
-Add Autarch, mimes, solitaire...anything related to harlequins for speed
Orks
-dont really like them :P
Tau
-I personally hate them (u have no idea how many online matches I lost against Tau :P)
Sisters
-possibly make them into a complete Inquisitorial race, merge them (if possible) with the Ordo Malleus and Ordo Xenos. (u could ask permission to use them from mods that have them, again, just throwing in ideas)
Necrons
-Pylons
-not much, since u scared me with their Titan
Dark Eldar
-Webway gates (something like Eldar's only a bit....I dont know....Dark Eldar-ish
-possibly give wyches a random unit thingy, like FoK's Space Marine Squad
-add incubi as a separate unit from archon (and make archon attachable to other squads)
-make the Reaver jetbikes into a squad...its kinda cooler
IG
-tanks, tanks and more tanks
-conscripts (cheaper, less pop, bigger unit quad, ridiculously weak)