Unofficial Crossovers
In contrast with legal crossovers, unofficial crossovers are created solely because of the artistic pleasure derived by its producers. Generally, unofficial crossovers take the form of fan-written fiction and fan art, but it is increasingly prevalent in amateur films and audio. Whereas official crossovers are frequently stymied by such concerns as copyright, royalties payments, quality of writing and ownership of the characters, unofficial crossovers are unfettered by such concerns, so long as property holders do not exercise their right to enjoin the distribution of such material.
A good example would be the unauthorised live action fan film Batman: Dead End, which brings together the properties of Batman, Alien and Predator in one setting.
In the film I Am Legend, a billboard can be seen in Times Square depicting a crossover film of Batman and Superman, with a release date of May 15, 2010.
Unofficial crossovers can also occur in a "what-if" scenario. The most notable is an episode of Family Guy entitled "Lois Kills Stewie" which turned out to be a simulation along with the previous episode "Stewie Kills Lois". Stewie Griffin is confronted by Stan Smith and Avery Bullock from American Dad! while hacking into the central power grid at the CIA in a latest plan for world domination. Stewie mistakes Stan for Joe Swanson, due to notable similarities between the two shows. Stewie threatens to turn off all the electricity in the world until Stan and Bullock fulfill his demands of being "President of the World", which they do. Even though this never really happened, this is considered by some to mark the first official appearance of American Dad! characters in a Family Guy episode. Roger makes frequent cameo appearances in Family Guy, while Brian makes cameos on American Dad!. Roger, Rallo Tubbs, and Klaus Heissler were seen in the final Family Guy Star Wars spoof, "It's A Trap!", as an Imperial Officer, Nien Nunb, and Admiral Ackbar, respectively. Stewie also appears as an interactive hallucination of Booth on Bones when the agent has issues over possibly becoming a sperm donor, with David Boreanaz (who plays Booth) repaying the favor in Road to the North Pole.
Sherlock Holmes's War of the Worlds introduces the famous detective to the scene of London occupied by Martian invaders, depicted by H. G. Wells - the crossover facilitated by the fact that both works, set in late Victorian London, are now in the public domain and can be freely used and modified.
Fan fiction crossovers between different science fiction movies and series are often created, such as Star Wars vs. Star Trek or Babylon 5 vs. Stargate. M.U.G.E.N. is a fighting game engine that features many fan-created and fictional characters and stages from various television series, movies, as well as other video games.
Read more about this topic: Fictional Crossover