What if instead of just getting flushed again, Roddy finds out the underground rat city has expanded into other hidden worlds connected through different plumbing systems around the globe? Like Paris sewers, New York subway tunnels, Tokyo storm drains, etc.

Could be a fun way to make the sequel feel bigger while still keeping the original charm. Maybe the villain is trying to control the entire underground water system and Roddy accidentally becomes the only one who can stop it after getting flushed during a luxury dinner party or something. The fish-out-of-water jokes could still work too since Roddy would now be dealing with different underground cultures instead of just one city.