Here are some ideas I have for combining different worlds, different storylines, and characters from different tales.
Macbeth and Star Wars -- Who can resist the Scottish Play with lightsabers? The Weird Sisters could be Sith Witches. The plays of Shakespeare have themes that can be adapted in an endless number of modern ways, why not set it a long time ago in a galaxy far, far away and watch Macbeth be seduced by the Dark Side of the Force? You laugh, but you know it would be cool as hell. It would, of course, need high production values to be done right.
Survivor and Lost -- For the next Celebrity Edition of the popular Survivor reality show, I suggest we get the cast of the ABC hit and drop them on some secluded island in Fiji. Base the real challenges on plot elements from the show. Winner gets their own spin-off series! I think Mr. Ecko would win the whole thing!
Medium and Ghost Whisperer -- Now that these two paranormal dramas are on the same network, a crossover seems natural. Have Allison Dubois with her precognitive dreams and hauntings cross paths with seer of dead people Melinda Gordon. It would be nice seeing the lovely Patricia Arquette act side by side with beautiful Jennifer Love Hewitt.
South Park and The Simpsons -- It would be awesome to see the two greatest animated comedies combine, first in the construction paper style of South Park and then in the unique yellow-faced design of The Simpsons. What mischief would Bart get into with Kenny, Cartman, Kyle, and all the rest?
Batman and Superman -- A live action crossover of DC Comics' greatest superheroes has been rumored for years. It happens all the time in comicbooks, so why not in a movie? At least have an episode of Smallville in which Clark Kent meets a young Bruce Wayne.
Let me know if you think of any others.