I wonder why no one has written this fic yet. The idea just popped into my head; I've read a story here or there that has the hand regenerating into Nine (which I told myself made sense feasibly because even though the regeneration is supposed to rewrite every line of DNA, heh, it can miss a few. Anyhoo, in my idea, the DNA strand that connects with Donna is Nine's, and so he looks like Nine but with Ten-ish hair.
What? Eccleston's had Ten-nish hair before. See? Well, sorta. But anyway, instead of, erm, NineII been sent off with Rose to the alternate universe, Ten, Rose and NineII "enjoy" each other's company aboard the TARDIS. Or, he can still have the close-cropped hair. That works too.
Yeah, just surprised this hasn't popped up anywhere since the hand regenerating into Nine is already out there.