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.