One spoilery thought:
We know now that Jack can father children, unlike the immortals of Highlander. Assuming his usual behaviour, and occasional failures in any contraceptive method used, how many generations will it be before all of the human race is some sort of great-great-grand-whatever descendent? My guess is that it's within the next few thousand years, but my maths isn't good enough. Certainly well before he becomes the face of Boe - is that why he's so interested in caring for the humans in New New York? And the odds are good that he's his own great great whatever grandpa many times over!
One crossover thought on this - if the Watchers have Jack spotted as an immortal, what the hell will they think about his children?
