Captain Jack Harkness is arguably the greatest male companion on Doctor Who. Every single viewer adores his performance, and he has not been on the show since around the airing of series 6, when he was doing Torchwood.

We as fans want him back, we want stories with him, and we want to see more of his character. The dynamic between him and the new Doctor would be intriguing and make for great story-telling. There is no reason to not bring him back, and I am sure that if he were offered the opportunity, John Barrowman would not decline it. Chris Chibnall could figure something out with the story - that's not the viewers problem.

Understandably, nobody - including the BBC - wants Doctor Who to exist purely to satisfy the fanbase. However, with something like a character that was never fully completed, it is completely reasonable to ask for his return.