Besides my 'Cold Diamonds' idea here's another one. How about one where Fry, Bender, and Leela are assigned by the Professor to deliver golden rings to the planet Mobius? Fry gets all excited because he knows Sonic the Hedgehog lives there so he invites the whole gang to Mobius to meet Sonic. But they end up in Robotropolis instead and have a run-in with Robotnik and pure hilarity ensues!
Also, Bender can get really pissed off with Robotnik for creating evil robots with no free will of their own, Amy can fall in love with and chase around Robotnik's lackey, and the PE Crew takes down all his robot armies! Let me know what you think! If you didn't like this idea, no big deal.