finally, a conspiracy theory that doesn't involve Skynet at all!
The man we've all been calling future!John, isn't the future all-growed-up John Connor at all. Since every temporal loop has to start somewhere, he gave himself the name of "John Connor" - before or after starting stories about such a great guy. or maybe he just started to be called John Connor by others who'd heard the stories.
Who is this guy? I don't know. Maybe he picked 'connor' at random...maybe Sarah was his childhood sweatheart, or they were in the loony bin together.
hmm...maybe it's really John's friend, the kid who's going to the prom with Cameron.