Hilariously, Parker and Stone offered Seinfeld the non-speaking role of "Turkey Number 2" in that season's holiday special, which Seinfeld refused (via The Independent). Along with the fact that the turkey is a background character with no lines, since the series is animated, there would be literally no way to know which turkey was Seinfeld, unless Parker and Stone were going to ask Seinfeld to make bird noises, which definitely would have made for a memorable episode at the very least.
Alas, since Seinfeld declined the offer, we never got to see what certainly would have been one of the most unique performances of his career, but the story itself demonstrates Parker and Stones commitment to poking fun at any and all celebrities. Even when the king of the '90s offered his talents to the series, "South Park" just used the opportunity to make fun of him straight to his face.
