It's happened before, and it'll happen again; announcing a new Doctor just wouldn't be complete without the bookies offering odds on who'll be the next one. Presumably we won't be given a hint until the November anniversary at least, so there's plenty of time for speculation.
The Radio Times is reporting several 12/1 picks: Rupert Grint (Harry Potter), Rory Kinnerar (Women in Love), Chiwetel Ejiofor (Dirty Pretty Things, Love Actually), and David Harewood (Homeland). Naturally, actors from other BBC dramas have been suggested as well; Colin Morgan (Merlin) is at 16/1, and Benedict Cumberbatch and Martin Freeman (Sherlock) both have odds of 25/1. There's good odds for women taking the part as well. Miranda Hart (Call the Midwife) seems a long shot at 33/1, but Zawe Ashton (Fresh Meat) and Dame Helen Mirren are both at 20/1. The Hollywood Reporter quotes Russell Tovey at 10/1 odds (RTD's own suggested candidate for the Eleventh Doctor), and tossed several other names into the mix; Ben Whitshaw, David Morrissey, and Helen Mirren, among others.
Meantime, the BBC is noting some of the sillier candidates - Tom Cruise, David Beckham and Simon Cowell among them- and some actors who already have links to the series. David Warner has already appeared on the show, and Sean Pertwee has an obvious family connection. Most likely, we'll see another fairly fresh actor like Matt Smith, as the Guardian suggests, but there's no harm in guessing.