7. Archduke Franz Ferdinand
If you're looking at the heading and saying who, then you haven't studied the causes of the First World War. That's precisely the reason that the Doctor should meet him. The causes of WWI are extremely confusing, even for historians, but the actual declaration of war hinges around the assassination of Archduke Franz Ferdinand of Austria. When studying WWI in year 9 at school, we watched a video about the assassination and I can clearly remember the crowds of people. I can imagine the Doctor being one of them, and sadly realising that they'd chosen the wrong year to celebrate Serbia's National Day. I know that Ferdinand and his wife, Sophia, visited a hospital during their tour, so what if they met a mysterious Doctor while they were there. He would be determined to give them the best journey of their lives, and collect their children, so that they can, unknowingly, have a final happy family memory together. So he takes them to some amazing planet or maybe the Archduke's favourite historical period, and then some aliens come along and spoil their fun. But even though they've had to face a death defying situation, the whole family loves it and they go back ecstatically happy. This would obviously have a bittersweet note, but if the BBC is reading this and get their act together, I'm sure this could be aired on 28th June next year, then it would mark the 100th anniversary of the event!