Second most popular new race after Necrons is Chaos. Some units I would like to see from Chaos are:
Hellbrute, possibly a line unit as a Chaos equivalent of Space Marine Dreadnought.
Possesed, definitely a Elite melee unit squad.
Warp Talons, also probably a Elite, except with a jump jets.
Forgefiend and Maulerfiend, perhaps a Elite too, as a Chaos equivalent of Space Marine Venerable Dreadnought, Ork Mad Dread and Eldar Wraithlord.
As for Super Units, probably a Greater Deamons since adding a Renegade Knights would take a unique feel of Space Marine's Super Unit.
Of course splitting Chaos arrmy into Chaos Space Marine and Chaos Deamon in game, just like GW did in tabletop might be interesting, but does each of the two has enough units to stand on its own in game? That's a tough question.
Feel free to post your own thoughts about it. More Future factions post coming soon.