We do know that all the major factions have no problem wiping out another race. There probably used to be many more sentient races in the galaxy. But if they weren't space-faring, chances are they were absorbed or destroyed. The Hrud are space-faring but control even less territory than the Tau. They are apparently well consolidated to hold their territory. The Barghessi seem to be confined to the Grendl Stars, implying not space-faring. I would have liked to see them accompanying DE Beast Masters(it's in the fluff), but oh well. So most other sentient life is likely gone or part of another force. Grey Knights have Jokaero; Tau have Kroot and Vespid; and Dark Eldar have Sslythe and a few other mercenary races hanging around.
More races and even Ad Mech would be nice. But with so much out there already, GW runs the risk of violating another IP. And then there is the great and powerful "codex creep."
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."
Last edited by Archon Dan; 05-21-13 at 05:34 AM.