But the characters' comebacks would have to be "seismic," says Moffat.
Where have these 2 characters been over these years? It's time they were due a comeback. Both River and Jack are yet to meet the 12th Doctor (Peter Capaldi) in fact Jack never even met the 11th Doctor (Matt Smith).
"Seismic" returns for River Song and Captain Jack Harkness would be the only way the deceased Doctor Who characters could come back to the show. - Moffat
But that doesn't
mean that they won't return though! River Song was last seen in 2013 in "The Name of the Doctor" as a computer projection. Whereas Jack has disappeared from Doctor Who since 2010! This was during the episode "The End of Time, Part 2" where he was seen drowning his sorrows after the events of Torchwood: The Children of Earth in an intergalactic bar.
Moffat told Zap2it that the characters' return is,
"never, ever closed off. It's all about 'Do you have a story that's worth it?' Unless you've got a story that is absolutely informed and invigorated by their presence, just bringing a character back in an ordinary story won't do. Both River Song and Jack Harkness are massive characters in Doctor Who mythology. Their returns would have to be seismic, not routine." - Steven Moffat
In the terms of River Song returning, Moffats comment was slightly different. As in the Doctor Who Magazine:
“My immediate instinct was that story’s probably done. Not that we saw all of it, but I never thought we should see all of it. I’m slightly tempted, because I imagine Peter Capaldi and Alex Kingston would be absolutely hilarious together…” - Steven Moffat
So whether or not River Song and Jack Harkness is up to Moffat. As like he says the doors are never closed!
Series 9 of Doctor Who returns this Autumn.