“Do you feel that River Song’s story is done?”Showrunner replied:
"Well, yes, that does seem like the end to me. Except, of course, it isn’t, and can’t be. There’s always the chance that River will show up again."
Speaking in the latest issue of Radio Times magazine, Moffat admits he has “specific ideas but not a specific actress" in mind, but says "A new companion gives us the chance to relaunch the show.
The Day of the Doctor (50th Anniversary Special): Info and Interviews.
Interviews with Steven Moffat, Matt Smith, David Tennant, Jenna Coleman and Joanna Page