Perhaps Ruffalo could play an American teacher at the university that kept getting mentioned, or play an American businessman Mr. Park interacted with?
At the moment it’s hard to see how the American Ruffalo would play a character that meshes with the all-Asian cast, though there are ways that would make narrative sense.
Another possibility is that HBO’s Parasite could open up another story entirely with only some loose attachments to the core from the movie.