All this talk of Richard E. Grant reminds me of "Curse of the Fatal Death;" which reminds me of Hugh Grant; which reminds me of when I was watching The Pirates! Band of Misfits and realized that, with Hugh Grant as the Pirate Captain & David Tennant as Charles Darwin, you had both 10th Doctors in the same scene together.
I kinda like the idea of Richard E. Grant as an alternate universe 9th Doctor but I don't think they'll get nearly so fanwanky in a Christmas special. Especially not when the convoluted fanwank is what people complained about the most during Season 6.