Barrowman has said that he'd come back anytime he was asked but that the end of DT's era was probably also the end of Jack's involvement.
Ultimately, it's down to Moffat and whether he wants to or can find a way to make it work. He's not going to ignore the effect it would have on viewing figures and reviews - a boost in figures for one episode is not always good news if the reviews suggest lack of imagination in bringing back an old favourite as an alternative to genuinely new ideas.
Doctor Who has to keep a balance between innovation and familiarity and you only have to look at the mixed feelings about how often they should have a Dalek storyline to see that going over old ground is not to everyone's taste. As long as Moffat and Co can - and wants to - come up with a strong enough plot so that it doesn't feel like it was just cobbled together as an excuse to get JB on screen, it will be fine. What I wouldn't want to see is the series constantly looking backwards - especially if it dwells too much on the last five years or so. I want new villains, new planets and new ideas and if it does occasionally revisit old stories and characters, there's 50 years worth to chose from and a huge chunk of the current audience who will see those things for the first time whilst getting deeper into the Dr Who universe - with the added benefit of giving a nod to us old farts.