Spongebob Squarepants as Eric Cartman

Spongebob would suit Eric Cartman because they're the most recognizable characters in their shows, rivals to Squidward Tentacles and Kyle Broflovski who despise them, and have pets who meow (Spongebob - Gary, Cartman - Mr. Kitty). Also, their laughs are famous, and they're vain, nonsense, idiotic, and foolish.

Any more ideas?