A few have said they wouldn't want the next film to include the theater since it was in the last movie. But I associate the theater with The Muppet Show. Including the theater wouldn't make it a direct sequel to The Muppets or It's a Very Merry Muppet Christmas Movie. Also, both movies that included the theater involved the Muppets having to save it. I'd like to see a movie that includes the theater, but the plot doesn't neccessarily involve the theater (like how the plot of Muppets from Space wasn't about the boarding house). And I don't want them to do another "save the theater" plot, at least not until we've had another seven movies.
But if the theater isn't included, I'd still like it to involve the Muppets putting on a show in some ways. Though I don't think I'd want them to be portrayed as big celebrities like in the last film (or in real life). But I also don't want it to be about them wanting to make it big in show business (like The Muppet Movie and Muppets Take Manhattan did). Maybe they could be actors who have had a bit of experience in the past, but haven't exactly made it (like a group of character actors).
Somebody pointed out that the Muppets already did the "guest star is evil" plot with Glenda Jackson and Heather Locklear. Now, I don't agree about the Heather Locklear episode, since she was under a spell that switched her from good to bad and she went back to normal at the end (and I haven't seen the Glenda Jackson episode so I don't know if that has a happy ending). But also, those two were real people, my idea of a guest star who ends up being a bad guy (whether he wants to use his celebrity status to take over the world, rob the Muppets, or some other motive), I was thinking a made-up celebrity as opposed to a real person (though they'd most likely have a celebrity play the "celebrity"... Though it would be funny if there was a new Muppet character who was meant to be the guest star).
I think Dr. Tooth said he'd like to see the Muppets as themselves in a different time period. I'm a little mixed on that, mainly because I'd like to see the Muppets in their regular clothes as opposed to period costumes. Not sure if that would take away from anything... But they also don't have dress like whatever decade it is. And technically, there have been movies taking place in the past (The Muppet Movie, Kermit's Swamp Years), even if the exact years/decades weren't mentioned directly.
You know, it might be great to see a movie where the Muppets go on vacation (though Muppets from Space takes place during a vacation Kermit takes). That hasn't been done in movie form yet. Heck, The Muppets ar Walt Disney World is the only special where they've gone on vacation.
An idea I had today, though it would probably be too much fan service, would be to include a number of Jim Henson Hour characters working for the bad guys, them turning against the Muppets because they haven't worked with the Muppets since cancellation. I'm thinking Digit, Leon (if Kevin Clash can be available), Vicki, and Lindbergh. The Gorillia Television crew would be good, except they didn't exactly work for the Muppets and wouldn't need revenge.
But whatever the wait is, I'd like there to be a new special or two before the next movie comes out (reviving the Halloween special seems like an obvious choice). Maybe have it written by Jim Lewis and/or the writers of Letters to Santa while Bobin and Stoller work on the movie (I wonder if those two would work on a Muppet special). And we need viral videos to return (well, they sort of did return with Kermit's Party).