I was thinking about this today and came up with a few ideas as to which games these characters could fit into:
Snoopy and Woodstock - Chip and Dale or Klokked: Charlie Browns adventure
The Charlie brown game would work because they were Charlie Brown characters. Also they were both animals in childrens cartoons, like Chip and Dale, so that could work. Also, I would rather see them throw things at people than see them hit people.
Beavis and Butthead - Bart vs Space Mutants, or Bart vs World, or a Double Dragon game
Bart's cartoon and the atmosphere in which it took place was similar to the ones that Beavis and Butthead came from. The towns look similar too, and one of the games has a lot of talking, so that could be hilarious. It would be cool to see a 2 player hack made. Double Dragon, with them, would just be funny.
Doug - Nightmare on Elm Street
There was an episode called "Doug's Nightmare on Jumbo Street" where he was afraid to watch the ending to a horror movie. It featured a character called the "Abnormal" who could be the boss of the game. He could be re-designed a few times to suit the Freddy Krugers bosses in the game. They could also maybe include some other Doug characters as enemies such as: Roger Klotz, Klotzilla, Wack Weatherman, Dr. Decay, Robobone, Percy Femur and maybe even the mayor. Just a thought. It held 4 players too so maybe a few others could be added.
I did a coupe of sprite edits of Beavis and Butthead in NES form (top). I resized the rest, but they are not colored yet. The ones at the top were recolored using the NES color palette. Just wanted to give everyone an idea of what they could look like.