1. The Master And The Doctor Are The Same Person
Back in the olden days, The Master was a plotter. A devious man with an evil plan, cold and calculating, like the Doctor himself can be sometimes. And when Russell T Davies resurrected the second most famous Time Lord of them all, he became a madman (the Master that is, not Davies). Plagued by drums in his head and a psychotic need to control the universe, he was again parallel to the Doctor's post-Time War pain. What if they're both the same man though? The theory - the wildest Doctor Who theory on the internet - says that the Doctor could one day regenerate into a new form and assume the Master's identity. What we'd have been watching all these years is not two Time Lords, but one Time Lord at different points in his own timeline. That's enough to send any Gallifreyan mad, surely, and it just so happens that the Master is unhinged. So much of it wouldn't make sense, but if Moffat ever gets sick of the show, it's certainly a way to end it with a bang...