It's a nice idea, Christopher, but Moffat has explained why he thought such a thing wasn't feasible -- there would be the risk that the past Doctor would be more popular with audiences than the incumbent Doctor.
I understand Moffat's perspective. But I still think a smattering of past-Doctor specials each year would help to create additional content beyond what the main production unit can handle. And it's something that the franchise already has a lot of experience with, what with the many lines of Big Finish audios featuring various Doctors ranging from Tom Baker to David Tennant. Also, Titan comics has ongoing comic lines for Capaldi, Tennant, & Smith running simultaneously (and I seem to recall hearing that they wanted to add an ongoing Eccleston series as well).
Actually, I wonder if the problem isn't so much brand confusion as financial. Spin-offs often don't get the ratings of the parent show-- Agent Carter didn't get the ratings of Agents of S.H.I.E.L.D., right? (I seem to recall it was dubious that it would get a season 2.)
Shame that. Few shows have charmed & excited me as much as the bits I've seen of Agent Carter.
^^According to the rumour mill, BBC did approach Toby Whithouse about being an interim showrunner while Moffat took a break, which Whithouse refused. RTD wants no more involvement with Doctor Who and supposedly turns down an annual invitation from Moffat to come back and write an episode.
I have heard that Whithouse has turned them down, saying that he doesn't want to produce any show that he doesn't create.
Has RTD explained why he doesn't want to come back? Is he just out of Doctor Who ideas? (It would make sense that after 4+ years of running the show, he might be spent on that front. It took him no time at all to run Torchwood into the ground.)