Doctor Who has this thing about reusing actors. Colin Baker appeared the Arc of Infinity before being cast as the Sixth Doctor, Freema Agyeman was in Army of Ghosts, and both Karen Gillan and Peter Capaldi were featured in The Fires of Pompeii. So although Joe Dempsie has already been on Doctor Who (in the much maligned The Doctor's Daughter), that doesn't take him out of the running by any means. He played the affable, loyal Gendry on Game of Thrones, but we don't know if/when his character will be showing up again, so it seems like he'll have an open schedule coming up. We know that Moffat prefers his male characters awkward, bumbling, and utterly dominated by the women in their lives. But it might be a breath of fresh air for there to be a strong, confident guy on the guy. Because hey, men have had a full fifty years of watching the girl companions in impractically short skirts, this is the least the female fans of the show deserve.