For a crossover, I once rewrote The Land Before Time VII on this premise: the USS Enterprise is tasked with bringing a group of paleontologists back in time (since everyone knows the Enterprise is capable of that) to observe dinosaurs. The only way to do this without violating the prime directive, of course, is to go down looking like dinosaurs. Hilarity ensues when Kirk and Uhura are sent down to earth as Rainbow Faces, trying to monitor Littlefoot and his friends. Obviously Kirk narrowly avoids giving away their true identities, thanks to Uhura's swift actions on several occasions. In all, I just wanted to write a funny "behind the scenes" story about the Rainbow Faces and their origins, and Star Trek just seemed like the... logical crossover. Unfortunately, the manuscript has long since been destroyed. Sorry