The title says it all: take two things that you love that are books, games, movies, etc, and make a crossover. Be Creative! Create backstories if you want! an example:
The time machine had been finished, but the results were not as Bently had expected. He was in a city; dark, damp, and smelling of clay. As soon as he landed, he saw a figure about twice his own size, wearing goggles. A snarky sounding voice came from the figure's shoulder: "So Jak, what do we got here? Another of your friends?" Coming this summer: Sly Cooper meets Jak and Daxter
