The Doctor's brother has a long and fascinating history strewn across multiple timestreams and media... Wait. the Doctor's brother? Yes, that's right. The Doctor has a brother who's alive and well and starring in lots of adventures over in the land of Big Finish. Irving Braxiatel - Brax to his friends - first snuck into the Doctor Who universe in the Seventh Doctor's New Adventures novel Theatre of War before going on to star in the Missing Adventures book Empire of Glass alongside the First Doctor. When Virgin lost the rights to Doctor Who, Braxiatel became a major recurring character in their Bernice Summerfield books and then transitioned over to Big Finish's audios and books along with the rest of Bernice's supporting cast. Since then, Brax's star has risen further and he's appeared alongside Romana in the Gallifrey audios. The Tenth Doctor even referred sneakily to Brax on screen once when he was asked if he had a brother. His reply of "Not any more" is seen as establishing Brax as a part of official continuity, although the Doctor clearly believes him lost in the Time War. If Gallifrey makes a comeback on screen, the door would be open to Brax's on screen introduction. Brax may have a long and convoluted history but viewers really only need to know he's the Doctor's older, more intellectual brother. This Sherlock/Mycroft-esque dynamic could work very well and since Brax's hands are not always clean, there is bottomless potential for conflict between them.