Squats! Or Demiurg, depending on your particular views on the subject. The point is that I've started converting a few squads of them from Dwarf Thunderers and various bits and pieces from my bitz box, to be used in my Imperial Army forces. The plan is to use them as veterans in carapace armour to represent their hardiness and also to crew any vehicles I include in the force.
The idea of including Squats in my force came about with the release of the new fantasy Dwarfs from GW. The new models are gorgeous and were more than enough to tip me over the edge of starting a Dwarf army, but I couldn't really justify starting a new Fantasy army when my Empire and Dark Elves were still unfinished. I was going to wait and start Dwarfs at a later date, but then someone down my local club suggested that the Burlock Grimmson character would look more at home in 40k than fantasy and something clicked in my brain. I could have my dwarf army, only instead of fantasy, I could tie them in with my Pre-Heresy projects. The idea just grew from there
I'm having a lot of fun putting together my Imperial army contingent. It gives me a lot more creative freedom than a standard IG force does, as the Imperiums force were a lot more open minded and diverse during that time period. I'm thinking of adding Beastmen after the Squats are done. For now though, the Space Dwarfs are my main project.