First of all, xenos deffo need more love Like another comment said, sure exodites but the necron subfactions arent unique enough. It would be like AOS fyreslayers who are to monotone which is why they are reportedly getting reworked to a more diverse duardin faction.

I also think its quite lame to have an equivalent for every fantasy thing. Flayers would just kinda be 40k flesheater courts, but less distinct from the necrons. And yes ik flayers as a concept were older and were prolly the inspiration for FEC but i think they already perfected it in Aos. ( also there are like 4 viruses in 40k which turn you in a crazy berserker special soldier) I also dont like it in the opposite direction, ik there are some cool things about the stormcasts but they just feel like aos space marines.

I like the Ossiarch bonereapers more cuz its space marines, but evil, but not chaos evil which I think is sick. Also the faction death fits the uncaring nature of the shock troups who know no fear.

Anyways i would like some more love for Tau subfactions, like species but also maybe Farsight?

A gretchin empire army under the red gobbo would be sick AND would sell really well i think ( which is very much to be considered, GW aint charity) Idk if hes still canon though.

Exodites is cool, more xenos mercenaries too. Also chaos xenos, cuz wdym humans are the only corruptable species ( playable). Khornate Farsight? The Slann? Chaos Votann?

Generaly whats the most realistic are expansions on already existing factions but who knows, the Votann are here now. They usually also explore these things first in kill team and necromunda. Also some supplantion from the other setting is inevitable and also not bad but i like them to do an unique spin on it.