It does sometimes get populated after a century or two. If a dothraki gets a kingdom title he seems able to go a-conquering. In my current game it's been 150 years since CoK and most of the Dothraki Sea is held by a single Khal (though his primary title is the Kingdom of Sarnor, it appears as his dynasty name).
My wife fled to his court after I tried to have her arrested (we're Targs and she was readying a claim host) and I was pleasantly surprised to find I could dragon-conquest the Dothraki. Locked her up, then gave the Khal back his independence so he could continue menacing my Ghiscari cousins and went home.