Elite would firstly be from Nurglite force but special could totally be Mercenary coming from other faction:
Chaos undivided:
Renegade Guardsman with Melta/Flamer/Plasma: Would work as a Flamerat
Renegade Ogryn with Rippergun/Blood axe: Could work like a Marauder with greataxe
Renegade Guardsman with small weapon (Autogun/Lasgun): Work under Patrol or static guard and will hold their position
Rogue Psyker: Use the warp to annoy us (pyro/Biomancy/and Chaos magic)
Beastmen: Strong elite (exist for the 4 Chaos god: Bloodgor/Khorngor, Tzaangor, Pestigor and Slaangor)
Khorne:
Blood Pact Guardsmen: Similar to guardsmen but work like the Berserker in VT2
Gore Mage: Invoker of Blood Wolf and/or other Blood magic
Blood Wolf: Dog that jump on their enemy (Fast but weak)
Blood/Chaos Commissar: Provide buff to nearby enemy (Like the Beastmen Banner)
Tzeenth:
Psyker: There exist even more pyker possibility for Tzeenth
Chaos Mutant: Tzeenth is the ChaosG with the most mutation
Bird headed Cultist
Slaanesh:
Screamer: work like the Witch in Killing Floor 2 (moved from Tzeenth as Slaanesh doesn’t has that many obvious choice)
Sound Cultist: Cultist/Guardsmen with Sound Weaponry
Then they could also introduce the Demon of all the ChaosG and next Ganger since they usually work with shady people and can be easily corrupted to chaos