4. Idris Elba
Idris Elba is one of the names that always seems to come up whenever they're casting a new Doctor. And really, why not? He's well-known, well thought of, and immensely talented. But I've always thought that he would be much better suited to the role of the Master. He doesn't seem to have the natural eccentricity that I like to see in the Doctor, and his intensity would make for an incredibly compelling villain. The Master has sadly become something of a laughing stock over the years...I mean, he's sort of like Wile E. Coyote, with all his ridiculously convoluted schemes that never work out. Idris Elba could go a long way in making him a credible threat again.