My re-imagined 9th Doctor would mark something of a turning point.
After many adventures as Doctors #7 and #8, the Doctor would be forced to regenerate by an incident of her own making. An experiment gone wrong and lo, time to change faces. What experiment? Doctor 8 becomes more and more obsessed with rewriting her biology so as to maybe do away with the process of regeneration or at least fine-tune it so that it avoids a change in personality. In her research, she learns something that she had never known before: Gallifreyans naturally change genders upon their sixth regeneration cycle. But, much worse: they only have 12 in all. Horrified at the lack of control over her nature and at the realisation that she has already used up 8 of her lives, she creates a machine intended to overwrite her genetic programming, but that instead kills her and triggers regeneration number 8.So is born Doctor number 9, played by Olivia Colman.
Her Doctor would be quiet and melancholy, sometimes stern yet sometimes warm. This is a darker Doctor, who has become slightly disillusioned with life and decides to travel the length and breadth of the Universe to truly understand what she stands and fights for. She would spend most of her time travelling alone, becoming a much more mysterious Doctor, save for when she needs the help of her Perambulating Holographic Light Interface (Polly for short). Gradually, through her philosophising, she would gain a greater understanding of the world, of the preciousness of life and of her own place in time and the universe: not as their master, but as integral part of them.