Moffat then went on to talk about what Russell T Davies, the series' ex-showrunner, had to say on the matter:
I mentioned in passing to Russell that we were probably done with River, he said, 'You can't be done with River! No, no, no. Capaldi and Kingston, it's a sex storm!'
So if you see an episode called Sex Storm by Russell T Davies in his post-Cucumber years... [there will be] lots of editing from me saying ‘We really can't do that, you've forgotten which show this is….’Moffat also explained why River is less likely to return,
It's certainly not ruled out. I have a sort of worry about keeping anybody around in the Doctor's life for too long. Because he's the Man who Leaves. He's the man who outlives everybody: that's his story. In the end it's the boy in his box, and he's alone.River, who is played by Alex Kingston, last appeared in the 2013 series seven finale.
[Source: Radio Times]