Will it be something historic to the show, like William? Or Sidney? Will it be something a completely alien name we've never heard before? Like... er... Will it be an in-joke, like John Smith? Will it be self-indulgent, like Steven? Anyone wanna take a guess?
Clara: "I know the Doctor's name." River: "Only they don't have a word for 'doctor,' because nobody ever gets sick here." *dramatic pause* River: "It's me. I'm the Doctor."
Arthur Dent. Merlin. Roger Rabbit. Optimus Prime. Bruce Wayne. Aslan. Kahless. (a) Prophet. Q. Sherlock Holmes. Taliesin. Saint Patrick. Elminster.
From Clara's reaction in "Journey to the Centre of the TARDIS," it seems like it must be a name that's recognizable and meaningful to a present-day human. Which seems to limit the possibilities. Actually Merlin seems like a good guess -- it was suggested in the original series' "Battlefield" that a future incarnation of the Doctor would be Merlin, at least in an alternate universe. Although if they went that route, some fans would inevitably accuse them of ripping off the Colin Morgan Merlin series, because some fans reflexively see any similarity as theft even when it's totally ridiculous to do so.
Doesn't want Sexy to know about Nimue for fear she'd mock him over the whole "locked in a tree" bit? (In what seriousness the question deserves because Emh is right [about it being Fred, of course!], Christopher's point about how Clara reacted is what I though in that scene.)
Just being silly for a moment... We've misunderstood all this time. He announces it all the time. We've simply thought he was stating a title "the Doctor" when in fact he's been saying "I'm Thehdahktor." Sincerely, Bill