I'll bite - what's your fan theory on the Doctor's regeneration?
Well I'm not sure if what you would call it exactly. A fan theory or whatever. But in my belief the next incarnation of the Doctor is dictated by the Doctor's subconscious mind at the moment of his death (which is kind of what we've confirmed with Peter Capaldi I think). If you actually look at where the Doctor was before and after each regeneration there is a noticeable pattern in what he turns into.
1. Throughout Hartnell's era he never was the man in charge because he either had Ian or Steven to fill that role. He basically just got to be the glorified bus driver. But when he died he had Ben and Polly, who were two much younger companions. Therefore, he needed to step up and be the man in charge and be a more comforting paternal figure. Therefore, we got the much warmer and more directly heroic and energetic Patrick Troughton.
2. Troughton's Doctor was "executed" by the Time Lords and his face was intentionally chosen by them. He had no say in what he would become next. Which is why Pertwee and Troughton didn't like each other so much when they met in The Two Doctors. One represented what the other lost and what the other was forced to be.
3. Pertwee's Doctor fought desperately to escape from earth but when the Time Lords finally gave him his TARDIS back, he couldn't leave Earth for more than two seconds before he answered his phone and came back at the Brigadier's beckon call. One heart was tied to the family he adopted on Earth and the other longed to return to the stars. The result was then when he died he had to become the ultimate bohemian who belonged to nowhere and no one. Even among his own people he stood out like a sore thumb.
4. Tom Baker had either traveled by himself or with a small entourage of select individuals. But when he died he had to look after three people not one or two. And to make matters more complicated two of them were orphaned and the other one had suffered loss and was desperate to find her way back home. He couldn't afford to be the bohemian anymore. The result was he had to be in charge again but not in a stern authoritarian way like he had been before. He had to be someone they could relate to and the result was a younger, older-brother figure.
5. Throughout Peter Davison's era he was timid. He lacked confidence and authority. And the result of that got his companion Adric killed and as we saw in Caves of Androzani, literally everyone else. Even his own companion Peri almost died if he hadn't sacrificed himself. So when he died he needed his confidence back. His authority and the drive to get the job done no matter what the cost. The result was a man with the face of authority (Commander Maxil who Colin Baker had played earlier in Arc of Infinity) whose attitude was that of, "fuck you and your sensibilities I'm here to help out and you will appreciate me and my rainbow coat and you will like it!"
6. Colin Baker had suffered head trauma at his death and a sudden regeneration which he had no time to prepare for. The result started as a jumbled and at first amnesiac Sylvester McCoy. Hence his initial happy-go-lucky personality. But as his brain recovered, his darkness from his previous life had crept back and worse than ever. He had now become a cold and calculating chess master.
7. When Sylvester McCoy died his darkness had pushed everyone away. Even his beloved Ace with whom he shared a father/daughter relationship with. He had been murdered and was all alone. Therefore, he needed to become more affable again. Someone who could love and be loved. The result was a younger, friendlier Doctor and the first kiss in Doctor Who history.
8. Paul McGann's regeneration is pretty explanatory. Just watch Night of the Doctor. Bottom line, he actually consciously chose to abandon the Doctor mantle and become a warrior to fight in the Time War.
9. John Hurt had been fighting his whole life. No longer calling himself the Doctor and believing that part of him was gone forever. But at the end, he saved Gallifrey and as a result as he put it, "I am the Doctor once again." He also lost his memory with the timeline syncing back up. The result was that his conscious mind believed that he had murdered his entire race and the Daleks. But his subconscious mind remembered that he had been the Doctor again and that's why his next incarnation was a darker more tormented incarnation of the Doctor and not another incarnation of the Warrior.
10. Christopher Eccleston's Doctor had fallen in love with Rose Tyler and when he died realized that his darkness was getting in the way of a chance at happiness with her. That's why when he regenerated he turned into a man who could be her friendly, dorky, quirky, boyfriend and the two could live out their time together.
11. The problem was that David Tennant lost Rose Tyler. The result of which scarred him more than any other companion seemingly did because his entire creation revolved around a life with her. This caused him to transition into a morose brooding Doctor who continually lost everyone he cared about. By the time he died, he had died alone in the TARDIS with no one to turn to and no one to love. The pain was too great. The pain of losing his love, his friends, and his entire world broke him in the very end. So in order to save himself he regressed. He had to change into a man who could be a boy again. Someone who couldn't form easy connections and who could just live in the moment whatever that moment was without being scarred by the past. This was the pain that Eleven's entire existence was meant to repress.
12. In the end though, Matt Smith had a revelation. His world was not gone. His people were alive and out there and it was his destiny to seek them out and find them. He had a quest to complete and he couldn't complete that mission while being a scatter-brained puppy anymore. It was time to grow up and be a man who would do whatever it takes to accomplish the task given. But to also hold him to the mark and remind him not to be so caught up in the mission that he would abandon other people along the way. The result was the face of a man who he almost didn't save, Caecilius from Fires of Pompeii. An older, fiercer Doctor who could be the one to save Gallifrey from darkness.
