It’s been rumoured and I stress rumoured that during series 8 of Doctor Who, we will see a younger, recast Captain Jack. I’m extremely sceptical that Moffat would go down this route, seeing as we’ve just had the Doctor and River Song storyline.
But, With John Barrowman’s profile rising stateside a recast would certainly make sense. He has just been cast as a series regular in CW’s ‘Arrow’ so, his availability would play a huge factor in him returning as Captain Jack. Going with a younger Jack would also make it easier for the writers of Doctor Who and Torchwood, to keep track of the continuity, when/if Torchwood ever returns.
When Jack first met the 9th Doctor, he told him that he had two years of his life missing, Moffat could certainly answer that open plot thread by introducing a younger captain Jack.
But, who would be capable of playing a part played so perfectly by John Barrowman. For me, it’s an easy answer. Beau Mirchoff (pictured above). Now, Mirchoff is not a household name but he does bear an uncanny resemblance to John Barrowman. Beau is probably best known for playing the son, of Barrowman’s character in Desperate Housewives.