RTD does not seem very minded to bring back the Master but has left the option open.
The Master has become a bit of a pantomime villain in Who, so he would have to be re-introduced properly.
Nice though Sontarens are, the thing is that, at the end of the day, they are just another alien species. They are clones, true, but other than that they are much like Klingons.
Unlike the distinctive nature of the Daleks and Cybermen, or even the Nestene/Autons, there is nothing particularly unique about Sontarens.
So what I am getting at here is that if they do an episode which needs a sci-fi alien enemy, it might be nicer to invent a new one rather than use the Sontarens just for the sake of the past.
That said, they did that with the Slitheen, and I don't think they worked.