The only two detachments I am considering are going to be fluff-based, and will simply match other units in other codexes.
For instance, the idea I stole from Da Joka. C'tan plus converted Humans as Necron Cultists. use Codex: Chaos Space Marines, Daemon Prince rules for C'tan, Chaos Cultists as Necron cultists.
Or Overmatrix, unique Spyder, as an Ork Warboss. It transfers commands to the Mindshackle Clutch embedded in the skulls of captured enemy children to use as shock and horror troops against their own kind, aka, counts-as Gretchin. Makes perfect sense, if Overmatrix and the Control Module (Runtherd) die, the control over the children starts to fade, and they flip out and run away.