The Doctor facing his own mortality is an idea I find completely lacking in interest. Mortality as a literary theme can be explored in countless works of art, chief among them those that don't deal at all with genre conventions. Addressing this theme in a series which tells the adventures of a man who constantly regenerates when he should die, a series in which anything can happen and be explained away with just one or two lines of dialogue, doesn't sound like an exciting perspective at all. I'd rather have the series deal with themes that connect more naturally with its tropes, i.e. what happens to your sense of self when you keep coming back as a different man every time you should die.