My idea is (or would be) rather shamelessly stolen from a half seen episode of Jackie Chan Adventures. As best I remember, some of the elves are the Secret Service to "Big Red" and protect him as best they can from Daolon Wong (voiced by James Hong), who sought to steal Santa's immense good chi for himself. The head of security appears at Uncle's shop, requesting help (Uncle is very powerful chi sorceror and only magic can defeat magic!). Naturally since Jade knows that yes, Santa -is- real, not only does she volunteer Tohru and Jackie, but reassembles the J-Team. Since someone big is needed to fill Big Red's shoes, Tohru gets the job while Jackie, El Toro and Viper attempt the rescue. All the while, Daolon Wong is attempting to break into Santa's Workshop(?), which is like an immense snowglobe as the last defense the elves were able to setup before Daolong Wong kidnapped Santa back in this synopsis' second sentence.
D20 Modern probably but no guns, other than possible popguns or Nerf style ones in case a player wants to go John Woo, stick fights with giant candy canes or other standard Jackie Chan weapons, etc. Hrm. Taking a page from Calvin and Hobbes (and Kim Possible), there's a swarm of evil snowmen to wade through as well.
I might do a twist like on Robot Chicken and have the Easter Bunny as the mastermind (Kill Bunny) or even more bizarrely have the DBZ Christmas version ("Ah! My Dragon Balls!"), which had Rudolph shooting lasers from his nose.
I think there's an old RPGA adventure with Anti Claus, possibly a "Save Fluffy" one.