In a new interview with Gothamist, Steven Moffat was asked if any characters from the Russell T Davies era would return, he responded by saying he would love to bring back Captain Jack. See an extract below:

Will any of the Russell T. Davies characters, like Jack Harkness [John Barrowman], come back? There's no rule against that. People talk as if somewhere there's been some schism.

Well, you seem to have drawn a line. There isn't an intentional line at all. I do think there's a danger, if you're always harkening back. Are you saying the show used to be better?

No, just some of those characters are ones people are attached to. I mean, Jack, who I wrote in the show, I'd love to have him back. I was thinking he should really be here recently but he's busy [making the next season of the Who spinoff Torchwood for Starz].

And I mean there are references to Davies' characters—we just had a reference to Rose, in fact. In my head there is a continuing story. There's no idea that we're abandoning anything. There is an element that the Doctor moves on from people in a rather scary but inevitable way. He won't be nearby forever.

Read the full interview with Steven Moffat on Gothamist here.