I never, in my wildest imagination, figured how Stan and Eric would look in a manga. http://daisy44.deviantart.com/art/Stan-Kyle-Eric-Kenny-153223432 
features a couple of images.
Now I did try to imagine what Kyle, Stan, Eric and Kenny would be doing as adults. My guess would be loitering chain-smokers and alcoholics… (on the other hand Kenny might not live to be 26).