Years ago, I posted on here my guidelines for writing fan-fiction. One of the things I’ve touched on in those rules is crossovers. Crossovers are one of the most considered ways of creating a fic, as we all tend to imagine characters from one fandom interacting with another fandom. It is usually in the forms of a confrontation, or team up. Some even hook up one character from one fandom with a character from another fandom. What many don’t realize is the complications the can create under normal conventional circumstances, and thus creates a massive universe.
Now, I have stated that if the genre is science fiction, or fantasy, it can be easy to do a crossover. Science Fiction can include such elements as dimensional jumping, or time travel, to achieve the crossover. This allows for these one shot crossovers to happen, bringing to vastly different universes and characters to meet. I must cite the Judge Dredd/Batman crossovers as prime examples of this. Gotham and Mega City One are two distinctly separate location, and if they weren’t it is plausible to see Gotham as part of Mega City One in the future of DC’s universe. Since the universe of Judge Dredd has already acknowledge alternate universes, thus having Judge Joseph Dredd meeting Batman had a plausible explanation. While the two universes could remain separate, that could also reference each other, knowing changes in one will have little to no effect on the other.
If one were to use a fantasy element, like magic, to do a crossover, it adds some limitations. First off, characters are not always in control of the crossover. They can’t always choose when they return, or who takes part in the crossover. Add to it, that depending on the origin of the magic, these characters could exist in close proximity to one another, relatively speaking. For example, if a wizard on Discworld were to open a portal, and drag certain characters from Marvel’s 616 universe (the main one for those unaware of this), it is plausible that not much interaction can happen, other than a possible humorous encounter between Death and Death.
The difficulty with magic can happen if you infer that the two fandoms exist in the same universe. Years ago, I came across a Generation X/Harry Potter fan fiction that attempted to rewrite the fifth book including some of the characters from the group. This would, in turn, change how some events play out in the book, and even the author of the fic confirmed this would be the case. Now, Harry Potter’s universe states that magical world is kept secret from the non-magical world. Marvel openly acknowledges that magic exists. Other than this, this story works great, in my opinion. Now, it is possible that there is some explanation of how mutants are not known to the magical world, and that mutants don’t know about the magical world at large, but even a cursory explanation can address that issue.
The biggest difficulty with crossovers is when they happen in the same universe, when there is no unusual means for it to happen. These can be the most troublesome crossovers, since it means that one has to accept the fandoms already exist in the same universe. This isn’t an issue if the sources tend to be in the same genre, or are controlled by the same source. For example, there is a shared universe between a lot of the dramas on CBS. Starting with J.A.G., which then had a spin-off called N.C.I.S. This show had two more spin-offs, N.C.I.S. Los Angeles, and N.C.I.S. New Orleans. Now the N.C.I.S. shows have crossed over with one another, which is expected. N.C.I.S. Los Angeles, however, had also crossed over with two other shows, the revival of Hawaii 5-0, and the show Scorpion.. Later, Hawaii-5-0 had a crossover with the revival of MacGyver. This puts seven shows in the same universe. Any crossover that happens with any of these shows will have to include facts established in a shared universe.
There are at least two other shared universe, however, that truly show the dangers of crossovers, due to shows being in different genres. First off, let’s look at a ‘universe’ 'centered' around a certain character named John Munch. The character started out on the show Homicide: Life On The Streets. This show had a crossover with Law & Order. Later, Law & Order launched a few spin-offs, Law & Order: Special Victims Unit, Law & Order: Criminal Intent, Law & Order: Trial By Jury, and Law & Order: LA. These shows have also had crossovers to with another franchise set in the same fiction, the Chicago Franchise, which has Chicago Fire, Chicago P.D., Chicago Med, and Chicago Justice. As of right now, that is ten shows. Some characters from all of these shows have also appeared on, or had characters appear on other shows. These shows include In Plain Sight, Conviction, and Jo. This ups the count to thirteen shows. Add to it that the show Deadline is set in the same universe, bring the full count up the fourteen. Now, John Munch has also appeared on The Beat, Arrested Development, and The Wire. That makes seventeen shows in this universe. But there is one more, that complicates this. John Munch has shown up on X-Files, in an episode that predates Homicide. This make eighteen shows, but due to the contemporary science fiction nature of X-Files, as well as the massive scale that some events happen on, with a mass infection at the end of the ‘The Event’ season, would be hard to ignore, yet apparently happened around the same time as SVU’s 17th-18th seasons, and the 4th-5th season of Chicago Fire. Mind you, this little thing is nothing compared to the massive crossover mess caused by Disney
You see, Disney has had a long history of crossing over many of the shows it has its hand in. It has even been stated that all the Disney Sitcoms exist in a shared universe, but that can be argued when there is a lack of interaction between them. To date, this is a total of thirty show, and eighteen of them are linked due to characters crossing over and being on each others shows, as well as spin offs shows. Normally, this would be no problem at all, but one of those eighteen shows is Girl Meets World, which is the continuation of Boy Meets World. That show, with its past, ties in seven more shows into the shared universe, one of which is a Netflix production. This is only one of the problems with this shared universe, with a second complication being that they decided that one shows key point was that it took place at the White House, with a president that never existed in our own reality. This complication does make this whole universe a mess, especially if one show references history, and notes the lack of that president.
As you can see, crossovers are a tricky thing to work with. Since most fan fiction is not canon, and thus most crossovers are non canon, it is most times not an issue. The real issue comes up when the crossovers are officially done. Companies, like Marvel and DC, have the plausible excuse of multiverses, which can allow crossovers to happen, and allow for significant changes to pesky things like continuity, and established back stories. Television shows, and movies, use the idea of the dream sequence and non canon holiday episodes to get around the crossover events, if need be. Movies use the idea of alternate timelines when a crossover happens, or even call it an inside joke, or subtle nod.
The best thing I can say is if the crossover is to be serious, remember to do very thorough research. If you acknowledge the full background of the universe you are working with, it‘s a positive for the serious crossover. If you do the crossover for more comical purposes, then just do it. A silly little chance encounter can be humorous, and doesn‘t need serious research. If you work with something with established multiple universes, you have a little leeway in the plausibility department. Other than that, be wary if you are doing a serious fan fiction. While it is not canon, you never know what is going to come down the line. You never know if, or when, your fan crossover will become official.