1. Peter Capaldi (With A Goatee)
Back in the days before Peter Capaldi was announced as the next Doctor, a lot of people were saying that he would make a perfect Master. After all, the role he's most known for is a frankly malevolent politician, which isn't too far off the mark. Isn't it exactly the sort of thing the Master would do, regenerating into a doppelganger of the Doctor, just to mess with him? And we've already established that the Master seems to have a greater level of control over his regenerations, as he's able to take the form of others (as we see in The Keeper of Traken) and decides to pick a body similar in age to Tennant's Doctor in the new series. All he needs is the delightfully evil goatee Capaldi's been seen sporting of late, and we're good to go.