Steven Moffat has revealed that the Doctor will never appear in the spin-off show Torchwood, because it is not suitable for Doctor Who's audience.

Talking to TVLine, Moffat said:
"The Doctor could never go to Torchwood, Russell and I both agree on that. Doctor Who has a tremendous relationship with children in Britain. They'd want to watch Torchwood then, and it's not really a children's show."
However, he's stated that he wants John Barrowman to return to Doctor Who, and he almost had Captain Jack featuring in A Good Man Goes to War:
"I'd have [brought back Captain Jack] for 'A Good Man Goes To War', but John was busy doing Miracle Day... When we've got a good story, we'll do it. You can't just bring somebody back and say, 'That's a story'. I won't be thinking, 'How do you bring Captain Jack back?' I'll be thinking, 'You know what this needs? We need to bring Jack back for it'."
Do you want Captain Jack back in Doctor Who?