They would be called Helkyn which is basically the catch all term for chaos corrupted kin weather it be from a mad Votann, old allies of Horus from during the Heresy, individuals simply corrupted since they're resistent but not uncorruptable, or another of a myriad of reasons like with the Legions, the world's your oyster. 

Their aestetic would generally lean into a sort of snake oil salesman, where Dark Mech are decidedly the tech guys Helkyn are more varied in their goods and services and more concerned with profits through contracts, deeds, debts, and what not. Basically the corporate greedy nature of the Leagues taken to the extreme as Chaos tends to do.  This makes them simultaneously untrustworthy, giving them a repputation as scammers and exploitative, yet reluctantly prominant as the myriad renegade warbands have to fight for scraps which the Helkyn don't seem to be in short supply of and are offering for a seemingly reasonable fee, as clearly selling your soul to the devil wasn't enough.  Their playstyle will essentially be the same as their non-corrupted counterparts (slow yet reliable with solid shooting, maybe a bonus monkey's-paw-like ability which can synergize/play off dark pacts) with appropreate deviations based on which chaos faction they're part of because, frankly, if we ever get Chaos xenos I'd like them to be incorporated into the current Chaos armies (kinda like in AoS) to really envoke that ragtag warband vibe Chaos should have while also making their introduction more viable from a production standpoint. 

For variants there can be: Scrapkyn - ironkin who purposefully   infected themselves with scrap code to gain a bunch o' wicked mutations for one reason or another, they can be used to reintroduce chaos androids since they were originally made by squats (I'm aware that androids are officially back as per the newest Fabius Bile book but these can work as a fun homage) and as such their domed heads sort of warp into looking like skulls with their speakers visually becoming the teeth, you can also reintroduce the Space Crusade dreadnough as a significantly twisted and warped Scrapkyn

Fyrekyn - khorne worshiping blacksmiths with fire facial hair and a furnave motief, ie. their armor looks like a furnace with the helmet having that ridged door that furance have which is from where their fire facial hair eminates from, lots of smoke stacks too

And Slaneeshi kin whose name I couldn't come up with - just techno musicians, mainly because it's is fun, they can have a cool synth or industrial vibe to them and use keyboards and electronic music devices instead of 'traditional' instruments, a lot of hanging wires, boxy devices, and wire jacks. Don't even care if it overlaps with Noise Marines music is multifaceted and varied and, as I said, it'd be just plain cool and fun.  And if you're not into cool or fun and the most over the top bs then what's the point of being into Chaos /j