For my own Tzeentch Daemons force, I've kept to the basic models for the most part, so overall it's kept more of the traditional 'Fantasy' look, and instead I've "
40k'ed it up" by doing things like;
1. All the bases are built with a cinematic 'theme' in mind. For example, my large squad of "assault" Pink Horrors have modeled bases to look like they're just overran a
SW Grey Hunters squad, with things like a discarded bolter, combat knives, a couple helmets, a dead Space puppy or two, battered shoulder pads, etc... Another squad instead has bitz of dead Eldar, etc...
Makes it feel like they're actually fighting in the grim dark far future!
2. Some characters are converted with a very
40k 'feel'.
My one Tzherald, who's backstory is basically that he assumes a 'twisted & dark mirrored version' of the enemy leader is based heavily on a
CSM Termie Lord with added spawn & other various bitz, along with some green stuffing to give him a somewhat mutating/ethereal look & feel.
This way, I don't have to worry too much about altering the vast majority of the basic models themselves, but can still get a decent
40k'ish look to things. And they don't look as super out of place when they fight the mortals of the Old World either, than if I'd done say 'techno-daemons'... (just ignore those scenic bases is all!
)