Funny enough, I had an idea like this once.
It was a Pixar short which features every Pixar character ever made.
It starts with Andy peacefully sleeping with Buzz and Woody in his arms. The camera roams the room and we see the rest of the Toy Story characters. It them zooms in on Andy's door closet, and the door slowly opens. Out of it comes Mike and Sulley from Monsters Inc. They slowly creep closer to Andy's bed. He wakes up and sees a giant shadow, forcing him to scream in fear.
The light comes on and Sulley tries to explain to Andy that he isn't an evil monster, which fell in deaf ears. The scene is so terrifying that Buzz, Woody and the rest of the toys spring to life and yell "IT'S A MONSTEEEEEEER!". Andy then doubly panics as he sees his toys come to life.
The green army men then charges towards both monsters, while Buzz uses his laser to stun Mike. But during the struggle, Buzz falls out of Andy's room through the window and lands on a colony of ants. Flik from A Bug's Life screams "THE GRASSHOPPERS HAVE RETURNED!" and everyone panics.
Back in Andy's room, everyone is running around while Andy was cold, with a panicked look in his face. Out of nowhere, The Incredibles come crashing in and begin attacking Mike and Sulley, destroying everything in their path.
Andy finally gains the courage to run out of his and alert his mother. But then, he is stopped by Dug the Dog from Up. "Hi there! Are you a friend of the tiny mailman?". Andy then runs away and hides in the bathroom. But in the bathroom window there's a pelican, it's Nigel from Finding Nemo! He opens his mouth to reveal Marlin and Dory. Dory greets Andy in her whale voice "HIIIIIIII THEEEEEREEE".
"Stop that!"-Says Marlin to Dory. "Have you seen my son, Nemo? He's an orange clown fish like me and...". Andy doesn't even let Marlin finish. He runs out of the bathroom only to find that his whole house has been invaded by Pixar characters, including characters from the shorts (like the two performers from One Man Band that haunt him with a nightmare symphony).
The madness gets to Andy and he lets out one big scream.
Andy then wakes up, and realizes it was all a horrible nightmare.
"Andy, what's wrong? Did you have a bad dream?"
"Yeah, I dreamed that my toys came to life, then a monster came to eat me and superheroes destroyed my house!"
Remy from Ratatouille enters the picture. "There, there. It was just a dream. There's no such thing as monsters and talking toys. Here, have a warm glass of milk and a cookie. I made it myself!".
Wall-E then comes in and tucks Andy in.
"Good night, Andy. Sleep well!".
As Andy falls asleep, the camera pans out to reveal that he is sleeping inside of Lightning Mcqueen. He is traveling down route 66.
"Talking toys? Monsters?? What type of movies are they making these days?"
He then drives off to the sunset, affirming the viewer that the Pixar madness would continue.
THE END!
That's an awesome fan fiction piece!
