"The franchise is not dead, and we built the game in such a way where -- we made a rich enough universe where it would be quite easy to step back into that world and make something cool," the creative director said at QuakeCon. "So, yeah, that'd be great, someday."
It'll have to wait for the time being. id is entirely focused on Doom 4 right now and, speaking personally, I like to think another RAGE title wouldn't be the first project after development has wrapped up. Give us something else, even if, yes, I too would like to see more of that universe.