Allow us to Explain
If you still haven't seen the 50th anniversary Doctor Who special "Day of the Doctor," then be warned that things are about to get really spoilery. Seriously spoilery.
Good to go? Excellent. In "Day of the Doctor," we discovered that the planet Gallifrey and its rulers the Time Lords were not actually completely wiped out, it just looked that way to the Doctor and the cosmos. In fact, the planet was shunted into a pocket dimension where the Time Lords remain exiled to this day. So the Doctor now has the option of rescuing his people (if he can find them). Sherlock co-creator and Doctor Who writer/actor Mark Gatiss has suggested that it wouldn't be a good idea to do this and he's not alone. Many creators and fans think the Time Lords bring about story complications and make the Doctor seem less special, and that it was a good thing that former showrunner Russell T. Davies eliminated them when the new era of the series began in 2005.
But these same complications could also lead to fun drama for the rest of us. Here are some reasons why the Doctor may fear the return of the Time Lords ...