Let's do this one backwards, counting down to my favorite pick.
5. Catherine Tate
This is my least likely candidate. One, because I don't think the Doctor could change sex or race when regenerating. (Have we considered that perhaps Time Lords can only regenerate into what they already are? Perhaps black female Time Lord(ess) only regenerated into black females. Duh.) Two, because on the whole brain exploding and dying thing. But I have to give the option of somehow bringing Doctor Donna back because she was so fantastically brilliant! So, y'know, something could happen where 11 could fix her brain and she ends up becoming an actual Time Lord.
4. Tom Hiddleston
3. Richard Ayoade
2. James Nesbitt
1. Rupert Grint
I know there's other much more likely candidates out there, but I have no clue who some of these people are. I've never seen a Bond film in my life (and we're going to keep it that way no matter how attractive Daniel Craig is) so I don't know this Rory Kinnear guy. Benedict Cumberbatch is too busy being awesome. Daniel Radcliffe has a relatively successful career as it is and I think I would rather punch him than see him as the Doctor.
Let's just make this clear. If someone has already been in Doctor Who I'm going to say it is highly unlikely they would become the 12th Doctor.