3. David Morrissey
David Morrissey has already played (almost) The Doctor in the Christmas special The Next Doctor. In fact he merely had an identity crisis and implanted memories but this didn't stop fans taking to his performance and he is frequently quoted as saying he would take the part. The idea of a regeneration into someone who looks like a character we have already seen - Mary Tamm regenerated into Lalla Ward at the start of Destiny of the Dalakes despite Lalla having played the major part of Princess Astra in the previous story The Armageddon Factor. The Sixth Doctor himself, Colin Baker, also played a small part in a previous episode and we therefore have plenty of precedent.