1. Harry Houdini
Harry Houdini is actually an alien. I'm going to repeat that, one more time:
Harry Houdini is an alien. Wouldn't that just make so much sense? It could be a small scale episode, and for the record, I'm always on board for more of those. Give me tightly written character pieces about saving a few people over big budget save the entire universe storylines any day of the week. There's two ways you could go with this. Harry Houdini is an evil alien, who is using his powers to hypnotize the audience while he's performing magic, and eventually more of his people will come and be the new overlords of Earth. Or, Harry Houdini is an alien who doesn't know that he's an alien, because he's been raised by humans. He finds out when he meets the Doctor (and of course the Doctor tells him, so tactless), and has a big identity crisis. Then the Doctor could take him to see his homeland, which is a world full of people who can do what looks like magic, but Houdini decides that Earth is his home. I'm just brainstorming here, but you get the idea.