3. Ten-Eleven
There has to be a Doctor on Doctor plot that would gratify all Who fans of all ages. A good one that I wish would come to life is Ten seeing Eleven in the 50th Anniversary Special AND recognizing him. What possibilities... From Eleven quoting "the Doctor lies" to his former self, and, like all Who paradoxes, Ten quoting it back to Eleven at the end. Imagine what Moffat's mojo could do with a one-on-one Ten-Eleven encounter with 5 minutes screen time alone!