Doctor Who has a long running and rich history so it's not as though guest writers can do whatever they like, but some new blood is occasionally necessary. It keeps the show fresh and less bogged down in grand mysteries that have yet to be solved, and also allows for a bit of fun along the way. Of course, writing a Doctor Who episode that fits in with the overall arc of the show takes time and lots of consultation. Not every writer would enjoy the gig, but as Neil Gaiman's episodes have demonstrated, it's entirely possible to bring new people into the fold when they are available. In addition to Gaiman, perhaps writers like Warren Ellis, Kelly Link or Maureen Johnson would come up with some fascinating adventures? Some of the Big Finish writers could be incorporated, too, and while they are not necessarily known names, they are already well-versed in the show's rules". Collaborations between the show's usual writers and writers from the show's other properties might generate new ideas that make the show even more exciting.
Sara Habein is the author of INFINITE DISPOSABLE. She is a staff writer for Persephone Magazine, and the editor of the Word Riot blog. Her Doctor is number Eight, but the Twelfth is right behind.