One of the most interesting things about serials as that they run at the same time as several other serials. Many times, these are the literary competition, especially when it comes to television programs due to time slots. With other media, you compete for other people’s money, because no one has unlimited resources to keep purchasing novels and comic books as they keep coming out, not to mention the fact that you have limited time for that kind of reading. Webcomics try to help each other out via sharing links on their pages to get a collaboration on growing their readeships but the time issue is key, and when it comes time to helping with a kickstarter or buying merch the money isse becomes just as relevant. Still, if you really want to boost readership numbers nothing is as awesome as a crossover.
For the uninitiated, a crossover is when characters from two different texts come together for a limited amount of time, usually one installment. If there are multiple characters already in there and one becomes it’s own series then it becomes a spinoff of the original and this can eventually lead to guest appearances and more crossovers later on. Comic books are the king of this particular literary device and do some pretty crazy things along the way. Sure you have your Batman and Superman coming together to defeat the Joker with kryptonite or something ridiculous but there are some very cringe worthy ideas on the crossover front (check some out here http://www.complex.com/pop-culture/2011/03/the-10-most-bizarre-comic-book-crossovers/#gallery).
What makes the crossover awesome beyond anything else is that it fulfills a fantasy but also raises particular questions that nerds want answered. Consider that Spiderman and Wolverine are forced to team up but both are quite reluctant about the idea. Some time just before the team up or right afterwards, a fight between the two breaks out and we can see which one could actually beat the other. Of course, you really can’t have them in an all out brawl so they will just fight it out for a little while, both be cool and eventually reconcile. If you want to see what would happen in a no holds barred match between some pretty iconic characters, check out the good people at Death Battle over at Screw Attack who do this kind of thing with actual research http://www.screwattack.com/shows/originals/death-battle (Harry Potter versus Luke Skywalker was surprisingly awesome). When you get a super brawl out there you might get what is commonly referred to as an ultimate showdown of ultimate destiny, thanks to this cool video http://www.youtube.com/watch?v=xwBK31tC5QM (do not see the weird Adult version of it, the better animation is not worth it).
From a narrative perspective, crossovers are a big headache. It’s certainly understandable within a particular shared continuity, like the Marvel or the DC Universe, where a particular set of characters are simultaneously doing things. The problem is that things have to happen simultaneously between each character’s particular adventures. Consider this, there is one weird moment in the X-Men comics where Magneto goes insane and basically turns Manhattan into a huge concentration camp for non mutants. I don’t know many of the specifics but one of the things that a lot readers were quick to point out is that New York is the city of operations for a lot of people in Marvel: Spiderman, the Fantastic 4, some versions of Iron Man, and way too many B-listers. Some of these guys should have tried to stop Magneto in addition to the main protagonists of the X-Men. One of the big lampshade hangings to justify their absence is that these heroes are off doing their own thing but you see that they are in their own comics, still in NYC and none of this is happening or being acknowledged later if they just happen to not be temporally synchronized at the time. It gets even weirder with characters like Wolverine, that are so popular that they appear in multiple series and crossovers simultaneously. When does he have the time to be doing all these kinds of things? Superman in the 50s had a similar issue as he was in six different series being run at the same time. Sure, this was the Golden Age and narrative continuity wasn’t the biggest thing but current cartoonists do this way too much. Guess the appeal of character is powerful enough to alter the time space continuum so that he/she can be saving the multiverse simultaneously in various scenarios.
Crossovers are cool but they have to be used sparingly. Avengers the movie is awesome because it brought these iconic characters together and even had a mini ultimate showdown between the heroes. it’s why DC fans have been dreaming about a Justice Leageu movie for decades. The problem is that the individual stories have to coincide for it make narrative sense. That means that their own stories need to stop. What made the Justice League cartoon so awesome was that it served as a sequel to the Superman and Batman animated cartoons but those shows had already stopped way before. If they had their own adventures still running it would be a pain to keep track of for even committed viewers. And then it was a show about a team, rather than a brief glorious moment of characters coming together. The team dynamic was still awesome to witness as it changed over many installments but it lacked a certain pnanache that the crossover brings. In effect, it works best as a “one-shot” story that somehow stays in the main continuity of a particular title (the 90s Spiderman cartoon did this amazingly). If you want some even crazier nonsensical crossovers that go beyond the superhero genre then check out some fan fiction, you’d be surprised what people have come up with.