To be honest working on the assumption that it's someone who hasn't been mooted in the press then the list of suspects should be very small by now...
I don't think it'll be Capaldi, but I do think he'd make an interesting Doctor. I do love the idea that he'd be the death knell for the show though
much like casting a 26 year old would be, or a gruff Northerner, or probably going further back casting, McCoy, Baker, Davison, Baker, aw heck probably all of them.
The show will end someday, but it won't end because of the actor because they can just be replaced.