All these different races live, die, and interact in the same continuum, and this is a great opportunity to land the Doctor in situations that have not been explored. What happens when two or three different races converge on a resource-rich planet, and one of those riches happens to be the humans themselves? The Sontarans want to subjugate them, and the Cybermen want to upgrade them. Scarier yet, what happens when the Sontarans see advantage in working with the Cybermen? The Sontarans generate clones quickly enough, which could give Cyber-Sontarans amazing upgrade potentials. Of course, the Doctor Who universe (or multiverse, or metaverse, when you consider all of it) contains more than what's within time and space there are pocket universes (as seen in The Doctor's Wife and in the classic show's E-Space Trilogy, Full Circle, State of Decay, and Warrior's Gate), the interstices of the Void (from The Army of Ghosts and Doomsday), and the Vortex itself (as expressed in The Time Monster and The Two Doctors from the original and Father's Day in the new version... well, strictly speaking, every time they're in transit, the Vortex comes into play, but those episodes discuss specific aspects of it). There are more stories waiting to be told with each of these.
Michael Marcus is a game designer, author, and mad scientist living in Hamtramck, Michigan; his current project list include a series of comic short-stories collectively called "One-Punch," a book on hypnosis and language called "The Prometheus Codex," a collaborative game project called "Art War," and a fun spy story called "The Adventures of Jack Uzi" at http://tinyurl.com/JackUziChannel (for those interested).
