Keep in mind that 1) Miss Piggy would definitely be Susannah, and she's crazy in love with Kermit, so he could be Tristan, but also that 2) Tristan is a total badass and pretty much no Muppet is. 3) Tristan is also an asshole, and only those crochety old guys in the balcony are Muppet assholes, and they definitely aren't sexy enough. Also 4) Tristan could be played by a human, also 5) Kermit would make a really good Alfred, plus 6) it can't be Fozzie because he has to play the bear at the end.