|Image by AbyssWolf|
One of my favorite examples of this is the Super Smash Bros. franchise, which combines characters from many Nintendo games. I never owned a Nintendo system more advanced than a N64, but I was always excited to see the official art and fan art that surrounded each new title, as people drew these characters interacting. Watching them fight can be cool, but my favorite art is where they're just hanging out and being bros.
Both DC Comics and Marvel have teams composed of stars of their own comics: the Justice League and the Avengers, respectively. While not combining universes, these stories bring together characters with their own back stories, villains, and settings and combine them in interesting ways. From what I understand, the DC and Marvel stories were pretty self-contained before the comics for these teams introduced the idea that all of the stories were taking place in the same universe.
Another well-known crossover is the Alien vs. Predator franchise, which started out as a single comic but was so successful that it eventually spawned several more comic series, some bad movies, and some pretty good video games. The pairing of the two races as hunter and hunted has become so accepted that some fans don't realize that the whole thing started with a fortuitous crossover and took off from there.
Crossovers are a great way to justify writing fan fiction. If you set your fan fiction in a single world, then you're writing something that the creators of the show could have done if they had wanted to, but didn't. This is fine sometimes, but if the reason that you're writing fan fiction is that you wish that certain characters had ended up together, or that certain characters acted in different ways, or because you really think that the show could benefit from one more character who is an idealized version of you, then you're selling the original creators short. If, however, you take from several intellectual properties, then you are creating something that could otherwise never have been. In my mind, creating crossover fan fiction is also more creative and makes a better writing exercise.
Of course, I'm biased when I say that, as I have toyed with the idea of amalgam fictional universes as well. Several months ago, my friends and I joked about a "Battlennium Enternity" universe that combines every major science fiction series we could think of, and is exemplified in the phrase "They can't stop the long life and prosperous good hunting from being with you."
Do you have a favorite crossover? Do you have some favorite series that you think should be combined? Is your secret He-Man/Hardy Boys crossover entering its fifth volume? Let me know in the comments!