What about doing a fractured fairy tale? I've done this with 4th graders before--we read several fairy tales (or several versions of one, like The Three Little Pigs) and then the students write their own versions. One way I've done it is to have all the students write a fractured fairy tale independently, and then take several of the best ones and present them to the class. Then the class works together to revise/edit one of those, and changes it into a script by adding more dialogue, narrators, etc.
I like using a fairy tale because then the basic story is already there and everyone knows it. It's easier for the class to begin changing small elements, like setting and characters, while keeping the basic plot the same. Good luck!
