5. William And Kate
Yes, they're not exactly historical, but Wills and Kate are iconic celebrities of our time, and one day will rule over the British Isles. Actually I could see this as some sort of Comic Relief special - baby George could be taken by some evil alien who wanted someone else to gain the throne, or maybe they've infiltrated the monarchy by marrying Harry or something. The royal couple don't know what to do so they go to David Cameron, who uses the emergency number passed down from Churchill (and possibly Margaret Thatcher), which is the Doctor's number. The Doctor has to race against time to find the royal heir using some sort of detector ('it goes ding when there's...royal blood?') and they track him to Buckingham Palace, where the Queen brings out some royal James Bond style self defence, saving George before bathtime. Wills and Kate could ride in the TARDIS too, Charles and Camilla already have! So, even though they aren't historical, you can see that there's a plausible story with them. And it would be so fun to look back when William is king at his little cameo in Doctor Who, which would then be celebrating its 100th anniversary (we hope!)