Honestly, one thread is sufficient. Game of Thrones got one due to it being the most active thread in the TV Forum (aside from wrestling) and it being a very complex show that necessitates a subforum. On Game of Thrones, there are dozens of characters and story lines going at any given time. It is a show with many moving parts, each of which can warrant its own thread. Conversely, for the most part, Doctor Who just follows the Doctor and his companion. Yes, each episode could have a thread, each companion and each incarnation of the Doctor could have a thread, but those threads would be mostly dead (there's just not much to talk about regarding the Second Doctor or Tegan...heck, even a thread about more recent companions like Amelia and Rose would be mostly dead because there is no development for past characters). So yeah, discussion would be very limited to just a few threads. As such, it isn't needed.