Yes, I was thinking admech or maybe inquisitors would be next in terms of the amount of effort required, but that's still a ways off. New units/soldier types and craft are the most work intensive part of the mod and the main limiting factor on factions added - it's taken us years to get 2-3, and only 1 is truly "done". So we usually pick what allows us to recycle the most bits - IG we already had the basic soldiers in flak and carapace, plus the Valkyrie and Lightning tilesets when I started working on them. The Sisters we had their basic unit, but bulletdesigner is adding a bunch of new armors and some craft for them. Admech will be harder since we'll have to make new unit sprites, but we can copy bits of others, say the servo-arms of the techmarine and support servitor and the robes from the servitor.
That being said, any other faction probably won't be ready soon. It takes a few months to get enough resources to release even the most basic version of a new faction.