Loads of great ideas here, especially the thought behind the djinn and furnace kings. My two pence;
Beefing up the soulblight range. In the old vampire counts days there was a great range of stylistic and thematic differences between the different bloodlines, which sadly seem to have died out with just a few vanilla vampires and Carstein special characters remaining. No need to retread old ground though. I remember at the time of the Strigoi's creation the developers saying they looked at a vast number of concepts, including Thugee style death cultists, Voodoo practioners, giant gluttons and humane vampires with a soul (much like Angel or Brad Pitt in Interview with a Vampire). For two birds with stone you could rope the mortals of Shyish into the faction, to act as thralls thematically and give the vamps something to chew on in game. Just the one kit would probably do for the mortals, as for the aesthetics all I can think is lots of robes and a nihilistic vibe. Probably a bit too similar to the new nighthaunt range though.
Surely it's a matter of when rather than if for aelves of Hysh. I'd love to see them go down the angelic route with these, by this I mean the old school weird heavenly imagery. Gentle looking brutes with innocent cherub faces, Gemini style shining knights, burning wheels of fire....google "Bayonetta angels" or "orphan final fantasy" for a visual representation of what I've mentioned above.
I really like HiddenElephant's idea of a gargant army. My take on these would be visual similarities to the sky giants and storm titans of dungeons and dragons or Jotuns from Norse mythology; distant relatives of the gargants and holding them in disdain for their brutish ways. Maybe they were quite populous during the birth of the realms, but were hunted down and slain by those who mistook them for gargants or creatures of chaos. To avoid the elite 4/5 model army getting spammed by certain builds you could always chuck in summonable elementals to beef up the numbers (I'm thinking something similar to blue horrors). Great weapons of crystal or obsidian and you have a design that sets them apart from others of their allegiance. As for why they'd fight alongside other creatures of destruction...maybe they just don't care? The sky titans come down from their frozen peaks to crush a mortal city that threatens their existence and a bunch of Orruks just follow them there knowing there'll be a scrap at the end of it.
Slaanesh. Not exactly original....but more focus on sins of excess other than lust (again not exactly original). Maybe as part of his/her escape Slaanesh is torn apart, exisiting no longer as one entity but six lesser Gods that collectively form it's former power. Split them into embodiments of lust, pride, vanity, gluttony, greed and sloth (not sure that one fits actually). Vain, beautiful gladiators as foot troops (like a cross between wardancers and dark eldar wyches). Huge, corpulent monstrosities borne aloft on palanquins borne aloft by slaves hidden under masks of idealised perfection. Some weird mechanics to diversify the Slaanesh range beyond running really fast; another post over on the Chaos discussion board mentioned a giant mirror as terrain and foot troops that allow you to almost teleport to the mirror (almost like the Necron Monolith). I'm thinking design wise something akin to the more flamboyant elemts of Xerxes horde in 300...not just the orgy tent, but the weird execution beast, the exotic beasts with their handlers.
Oh and some sort of comeuppance for the former pretenders of Slaanesh (Outcasts of Slaanesh)? Maybe reduced choice in units and slightly lower stats as they feel separated from their God but more willing to ally with other Chaos. I've not thought that one through...
Well that was a bit longer than expected but that's what this thread is for I guess!