THIS IS ALL SPECULATION. THE VIEWS HERE ARE OPINIONS, NOT FACT. THEY ARE THE IMAGINARY DWELLINGS OF A USER AND SHOULD NOT BE CONSIDERED TRUTHFUL IN ANY WAY.
What if the boys grow up? Well, they won't but it's cool to imagine.
CartmanHis mom spoils him. Cartman probably is going to end up living in his mom's house forever because he doesn't have what it takes to live alone in the outside world.
Stan-First of all, no matter what your !@#$% ^&*() South Park fanon says Wendy and Stan will never make it. Since he is the most normal character, he probably becomes a middle class citzien.
Kyle-He might become a lawyer just like his dad.
Kenny-Kenny might finnaly die but if he doesn't, he is going to end up as poor drunken retard due to the fact that his family can't afford to send him to college or give him any help for a future life.
Butters-I have no idea. Butters is a very complex character.
Jimmy-He is handicapable. But not handicapable enough to get a good life. :(
Clyde-Since Clyde has never actually had any personality, it is hard to actually think of what he might become.
Craig-Because of his recurring visits to the principal, and the fact that he always flips people off and is mean to everyone, he might end up in jail.
Token-He's black and rich now, he will be black and rich later.
Wendy-Since she is a somewhat liberal she might become a politic.
Mr. Garrison-Due to the fact that all adults in South Park are idiots, he will continue to suck at his job until he retires.
Chef-He returns I hope