Jester David
Hero
As an idea, I like it. But I can't see them going that route, because that means that the long-term future of the show would involve the main character becoming progressively more and more insane. Given the lukewarm response to Capaldi's "slightly off" Doctor, and the outright rejection of Colin Baker's unstable Sixth Doctor, I would be surprised if they wanted to follow that path.
I would expect the next incarnation to be back to being a much more friendly Doctor, probably a lot younger, and quite possibly something other than white male. Oh, and I don't expect to wait very much longer for it to happen - even a regen in the Christmas episode wouldn't shock me at this point.
I agree, from a logistics standpoint, it would destroy the show if the doctors got worse...
The sad thing, is Capaldi really wanted to be The Doctor, and they've kind of put him into a corner in this season. I'm not sure people are likely this season as much because of the way he's written to be an arse.
Ironically, both Colin Baker and Capaldi's Doctor's were meant to have the same arc: begin as dark, unstable characters with a familiar companion and then soften over time. It didn't work for Baker at the time, likely because that kind of character growth was hard for television at the time and the show was struggling. I think it'll be easier for Capaldi to manage; we're already seeing a softening of Capaldi as he tries to go to the afterlife to help Clara.
Personally, I rather like the darker more pragmatic Doctor. It reminds me more of the early Doctors. There's less whimsy, less wibbly wobbly. It's showing you can present an erratic, funny Doctor without making him overly childish.