Hmm...I think Cam would have stayed over, and they would have stayed in touch, and kind-of together, after John left. John wouldn't have married Nancy, and wouldn't have requested really dangerous postings (my idea in r'verse being that at some point he decides he really dislikes his life, and so he tries to get dangerous, risky postings as a way of not having to deal with any of it). And then eventually he and Cam would have ended up getting assigned to the 302s together, because John would have been around enough for Sam to recommend both of them.

And then from that, it could go any way, really, from one or both of them being shot down and killed in Antarctica to them winding up together at the SGC/Atlantis. Obviously, I prefer to imagine something closer to the latter than the former :o)