Jacks revival being linked to the Time Vortext is one possilbilty, I wonder what would happen if Jack was to be somewhere with no time (and by extension no Time Vortex), quite how he gets to that place I dont know, but then who know there was a void between realitys that Void ships could hide in.
Bingo. The Doctor asked Jack if he wanted to die with dead seriousness. Jack said he didn't know, and then said seeing people like the ones trying to get to Utopia made him want to keep going.
I get the feeling the Doctor could end Jack's existence if Jack wanted it. If Jack had said, "I'd give anything to die" the Doctor likely knows how to make it happen.