3. The Storyline That Such A Thing Suggests...
OK, Just imagine this with me.. The Doctor regenerates and becomes evil. Bad things happen. And we spend the next three series watching The Doctor slowly learn how to be good again. Which means we get three series of watching The Doctor make choices that we've never seen him make before. See him do the things that we know The Doctor would never do, and then see him gradually learn again why they're wrong. It would be like (to use a contentious example) the Spike years on Buffy, watching him go from evil, to finding a reason to try to be better, to struggling to become a better person. Supposing he got Clara killed in his first evil scheme out? After the lifetime(s) she's spent saving him again and again. And supposing his first response was that he didn't care, only to have it eat at him for years until he finally redeems himself in her name? THAT would be some compelling television.