It's hard to write some one else's idea for a fanfic. But I bet that if you want to write a fanfic, there's already one inside you somewhere. Take a feel around your mind, it's there. Just think about it a little. Ask yourself a nice long series of "what if" questions. What if some one got hurt from one of Gonzo's acts? What if Floyd and Janice broke up? What if one day Statler and Waldorf stopped showing up at the theater? What if Elmo got really depressed all of a sudden? What if Grover got grouchy? What if Oscar got cheerful? (now there's a scary thought...) What if Travelling Matt got trapped in outer space? What if there was a huge drought in Fraggle Rock? What if Gobo got lost one day when he was exploring? Then ask yourself why those things would happen, how they could be resolved... you'll have yourself a story. Or you could start with the why. Why does Scooter look so different from his uncle J. P. Grosse? Why is Pepe obsessed with the word "okay"? Why doesn't Zoot talk? Why do the penguins throw each other in the air? Why can't Big Bird ever remember Mr. Hooper's name? Why does there seem to be such a surplus of children on Sesame Street? Why does Telly worry so much? Why doesn't Kermit go to Sesame Street anymore? Why does Red like to swim so much? Why is Boober so afraid of germs? Why... ok I don't know as much about Fraggle Rock...
All you need is a little one on one time with the voices inside your head. And don't tell me you don't have any, because we all do. Think about Muppet history, speculate on Muppet future, find the gaps, and fill them in. Don't force it- just let it come.
Oh, and BEAR? Good luck.