I don't think it will be Peter Capaldi, much as I would love it to be him, is that I think he comes with a bit too much baggage. A lot of adults knows him from TTOI - and can you imagine kids saying "I really like him, mum, what else has he been in?" 
I read somewhere that John Hurt is supposedly the 9th Doctor, a regeneration we didn't see, though I'm a bit confused on that as I thought it was accepted that Paul McGann was a proper Doctor? But I don't think it will be him either.
I wouldn't mind it being Rory Kinnear minor crush despite the fact that he's looking more and more like his father. Even if he is filming something, he's only got to film a short bit for the Christmas special, hasn't he? Like one day's work?
Rupert Grint - hmm, again too much baggage.
Chris O'Dowd would be amazing!
Ben Daniels is being rumoured quite a bit, isn't he? I could see that working.
I think it will be a relative unknown, though. Tennant and Smith were both small names at the time. I just don't see that a lot of the big names rumoured would, at this stage in their career, agree with the hassle and possible type-casting of being the Doctor. Plus, much as I love DW, it's not been brilliant the past season or two and it's not at the peak it was a few years back...