Russell T. Davies has revealed that new Doctor Who showrunner Steven Moffat wants to feature Captain Jack Harkness (John Barrowman) in the show at some point in the future. Davies made the comments in an interview with IGN where he also discussed how he and Moffat exchange storyline details on future plots on Doctor Who and its spin-off shows The Sarah Jane Adventures and Torchwood so nothing clashes between the three shows.
Speaking to IGN, Russell T. Davies had the following to say:
"Well, Steven said he'd love to see Jack in Doctor Who. So if Steven says that, Steven will make it happen, I would think. That's not inside information, but I bet one day it will happen. I'd love to see it. It would be marvellous."
The character of Captain Jack Harkness was first introduced in the first season two-part story The Empty Child/The Doctor Dances which was written by Moffat. The character appeared in five episodes of Season One and was subsequently given his own spin-off series, Torchwood. John Barrowman returned to Doctor Who as Captain Jack for the three-part series three finale and the two-part series four finale. He also had a cameo appearance in David Tennant's final story as the 10th Doctor, The End of Time.
As fans will already be aware Torchwood is due to return next year for a new ten-part series which will see Barrowman reprise his role of Captain Jack once more with Eve Myles back as Gwen Cooper. It's unlikely therefore we will see Jack back in Doctor Who anytime soon but with both Torchwood and Who filming in America next year who knows?