5. Captain Jack Was Beheaded By Headless Monks
One of the more straightforward theories that Doctor Who fans have come up with, Jack Harkness is rumoured to have been decapitated by the Headless Monks. It makes sense: Dorium was a 51st Century ma who knew Time Agents and dealt vortex manipulators, and Jack was a Time Agent from the 51st Century. Somewhere along the line, Jack became a head, and the Headless Monks are known for making that sort of thing so. The link between the Face of Boe and Jack was always blurry, but Moffat was the writer who introduced Jack, so why couldn't he be the one to behead him? What connects the idea further is the confirmation from John Barrowman that Jack was meant to be in 'A Good Man Goes to War', but couldn't get time off from filming Arrow. Maybe it's just as well: Jack would have probably made River jealous if he'd flirted with Eleven...