2. Susan Foreman
Well, stranger things have happened! Susan Foreman is deservedly cited as the definitive Doctor Who companion having been introduced as the Doctor's granddaughter in the very first episode, An Unearthly Child, back in 1963. As "the one who started it all", it came as a blow to long term fans that she wasn't included in the show's 50th anniversary celebrations in in some way, but her exclusion from the otherwise far reaching festivities has only made viewers even more eager to see her again. Her last regular appearance was in the 1964 serial The Dalek Invasion of Earth - in which she fell in love, and later decided to stay on Earth, with freedom fighter David Campbell - but the Doctor has yet to live up to his promise of coming back for her one day. It's
a pretty iconic speech, too, so he can't really expect us to have forgotten it. Over half a century later, now is as good a time as any to bring Susan back... and why not cast Maisie Williams to play a regenerated version of her? Maisie could be introduced as a character called Suzy and reveal her true identity in the final moments of a climactic cliffhanger ("Well, I couldn't keep calling myself 'Susan' now, could I...?"). Because
that's never been done before. Nobody would care, though, since it would be a moment of such momentous fangasming that it would well and truly rip the stratosphere of the entire fandom apart. You heard it here first, folks.