Before this film was announced I kept thinking of ideas for a theoretical Sesame Street film in the style of the new Muppets movie, though in reality they shouldn't copy The Muppets with their own characters (though interestingly I've always noticed a number of similarities between Muppets from Space and Elmo in Grouchland, both of which came out the same year).
My theoretical idea: A family decides to take their kid, who likes Sesame Street, on vacation to Sesame Street, only to learn that the whole street has turned into a dump and Oscar is the only resident still living there. He explains that years everybody else moved away and Oscar was free to have trash all over the street, enjoying it at first but then found himself missing the others, but hadn't done anything about it. He goes into his can and sings a song like "Pictures in my Head", maybe with some grouchier lyrics. The decides that they should go find the others and bring them back to Sesame Street, noting that he has all of their addresses.
They find Big Bird living in a nest on a tree in Ocean View Illonois, Ernie and Bert now run a Rubber Duckie Oatmeal factory, Grover is a waiter at a restaurant, and Cookie Monster has given up cookies. The Count, Herry Monster, Two-Headed Monster, Telly, Mumford, Prairie Dawn, and Snuffy are all located in a time-saving montage, and then they (and many others not in the montage) arrive on Sesame Street and start to clean up, while playing an '80s pop tune. Oscar is unhappy because everybody is back and the street is cleaner, yet is also happy at the same time because he's unhappy, and has just what he wanted again. Elmo makes a one-scene cameo, while Zoe, Rosita, Baby Bear, and Murray all only appear in a crowd scene at the end, Cookie Monster somehow goes back to eating cookies, maybe the gang has a telethon, and then they all sing a happy song, with cameos by Biff and Sully, Guy Smiley, Barkley, Don Music, Forgetful Jones, Aristotle, Chrissy and the Alphabeats, Bruno (being this film's Thog), Roosevelt Franklin & friends, and all sorts of classic characters.
The film probably won't be like that. I'll be happy if some old school characters make cameos or background appearances (Elmo in Grouchland didn't have that). I don't expect a Kermit cameo.