Things seem to be heating up a bit. With filming of the special to start in July, it shouldn’t be but a couple more weeks before Moffatt makes an announcement. You can guarantee one thing: no matter what Moffatt says, they have someone already cast, signed and ready to go. Jenna Louise Coleman had been cast, I believe, as far back as the early to mid 6th series but it wasn’t announced.
No one will truly know the answer until it’s announced, but I also know something else: Moffatt lies! So don’t take the ‘we haven’t cast it’ bull crap to be the truth.