In the tradition of all Muppets movies, one major character must be played by a human, with the rest of the cast filled out by Muppets. (Other humans may make cameo appearances.) I’ve illustrated this post with an image of Miss Piggy in a wedding dress (designed by Vivienne Westwood for the 2014 movie Muppets Most Wanted, per Vogue) purely for aesthetic purposes: I think Buttercup, as the straight woman of the story, should probably be the one character played by a human.
I think I might cast Gonzo as Westley. And I’ll leave the rest to you. (Of course, feel free to dispute my choices here!)
i saw the wedding dress at a Jim Henson exhibit at the Museum of the Moving Image... it's gorgeous.