I feel the show should focus on looking forward but I would not be adverse to seeing the characters again for Moffat's swansong - River was his creation after all, and Jack first appeared in a Moffat story.
If Series 9 is his last, it'd be a hell of a way to go. I'd have a two-part finale involving an assemblage of the universe's greatest time travellers, trying to stop a threat from the Weeping Angels...
Captain Jack: Series 9 would be a great time for him to make a comeback at any rate - marking the tenth year since the comeback of the show, he was the second companion-proper for Eccleston's Doctor. With Billie Piper already having returned several times, and Eccleston unlikely to be involved, I'd have Barrowman put on his long coat again and arrive via Vortex Manipulator again. It'd also be nice to know that his characters last scene in Doctor Who wasn't lamenting the death of his grandson in Children of Earth. It'd also give us a chance to ignore Miracle Day which I'm more than keen to do. It'd also be a chance to explore the 'two years' that Jack never remembered in his life potentially, if there was enough room in the story.
River Song: I really missed River in Series 8 though it's understandable the show is trying to move on to new things...as it should. But I'd have her back in a heartbeat, in a story involving a time-travelling assemblage at the very least!
Weeping Angels: They've not appeared since early in Series 7, and by the end of Series 9 it will have been a good few years since they were last seen. For the sake of a Moffat swansong I'd happily see them again in a decent story. Maybe tie their plot loosely into the ongoing Gallifrey plot. Just the odd mention, or maybe an affiliation with another Timelord they have going on? Keep them interesting, keep them scary but don't pull away from their basic tactics of time displacement.