Hmmmm... this is a really good question. A tough one too.
Liz: I could really see her going on to get her degree in a scientific field, so I think she would have ended up as a researcher or maybe even a professor. I have read a couple of good stories where she changes her career and teaches high school science, which also seems pausible to me. Another great story had her be a sci-if writer and I was totally into that too.
Max: I think Max would have stuck closer to home in the end, so having him became the owner of the UFO museum is something I could totally see. I also think if he and Liz stayed together, he might have taken over the Crashdown.
Michael: Again, I have seen many stories where he is an artist or a chef. They all work, in my opinion. Beyond that, I really can't think of anything else. Maybe a social worker of some sort because of his experiance in the system.
Maria: Again, I am not really sure. I agree that she could have become a singer. I've also seen stories where she ran her own business or work as a manager of a business, which works too. After all, she'd have first hand experiance with her mother's store.
Kyle: I totally see Kyle as a teacher. Yes, law enforcement is in his blood and it would be a really great career for it, but I've always seen Kyle as wanting to forge his own way, to break away from the norm. Both his father and his grandfather were in law enforement, so he might want to try something different. Beyond that, I think he'd want to share the enjoyment he got from sports to other kids, so a gym teacher is not too far of a stretch.
Tess: In a world where Alex is not dead or she is not responsible (and because I like her character), I can see Tess going in a couple of different directions. She has such a need for order and control, so some form of law enforcement is not out of the question for her. I also thing she might go the nurturing route because of her lack of, and become something like a social worker or stay-at-home mom.
Isabel: I very much agree with Izzy going into the medical profession. Also, stay-at-home supermom image or a teacher could work for her too.
Alex: He would rule the world with his computer skills.

Okay, maybe not the world, but I could see him running his own business doing programming or IT stuff for other businesses, building webpages, setting networks, and the like.
Beyond all that, with the amount of talent the writers on this board have, they really can make me believe anything. There are some stories where these characters doing jobs I've never imagined for them, but it is totally believible the way the story is written.