There’s a few things a writer has to pull off, IMHO, for a crossover to really work. First, you need an explanation as to why one group of characters suddenly shows up in another group’s setting, or you need an explanation why they were both in the same world all along. Bunnies will usually provide this with some mangling of reality. Second, you need to match tone. For example, I would never try to mix Cat Planet Cuties (the universe is friendly and wants to play!) with Tokyo Ghoul (cannibalism, ’nuff said).
Third and fourth, though – those are tricky. And pretty much interrelated. Third being “thou shalt not stomp on a character’s main schtick”, and fourth being “power levels must match, or this is a Curbstomp, not a fun romp”.
Rule 3 there means that if one of the characters you’re playing with is canonically the best hacker in the fictional ‘verse, then he’s the best hacker. (Unless your whole point is Battle of the Hackers. And I mean a real, evenly-matched, who knows who will win battle.) If someone’s Paper-Thin Disguise works on the characters of one ‘verse, then by golly it works on the characters of the other. If a horrifying monster – say, the man-eating shapechanging Parasytes, for just one example – terrifies everyone in Universe A, then it should utterly freak out those of Universe B who’ve just run into it for the Very First Time Ever.
Or, to boil it down, the crossover ‘verse must be internally consistent. Or at least a valid approximation thereof.
However, adhering to rule 3 can make rule 4 a bit tricky to pull off. There are only so many roles in your adventuring party! So… how to make it so crossover characters can both be key to solving your story’s problem, without anyone stomping on someone else’s toes?
For that, we can fall back on a nifty GURPS trick: base characters on point totals, rather than straight-up stats.
Power levels aren’t just, who can punch through a mountain. (Though that helps.) They can involve a lot of categories. Some I tossed with Kryal a while back include:
1) Experience and training. (Hello Batman.)
2) Organizational support. (See Stargate Command, or Gentleman Johnny Marcone’s mob.)
3) Mental resilience/grit. (Batman, Samuel Vimes, Shinichi Izumi in Parasyte who reaches new levels of Tranquil Terror.)
4) Knowledge of the enemy. (Ripley in Aliens.)
5) Damage output. (Punching through mountains goes here.)
6) Damage absorption. (Not breaking your hand on the mountain, here.)
7) Unusual background. Some aspect of the character that is either highly unusual or utterly unique to that character in that setting. AKA “You have a crazy backstory as a Phlebotinum Survivor or Adopted By Mountain Ninja Cult that justifies your Crazy Awesome skills.” See Cloud Strife, Harry Dresden, Harry Potter, Edward Elric (clap-alchemy), Blue Exorcist’s Rin as the Son of Satan, Aladdin as the Fourth Magi, and Ichigo Kurosaki as a living shinigami.
Note, most Main Characters should probably have a smidge of 7, or they wouldn’t be adventuring types to begin with.
But that gives you 6 categories to work with where the two groups of characters don’t have to be exactly matched. Maybe one group has great supernatural powers; but do they have a powerful government or Secret Organization that can make sure there’s a hideout, weapons, or help when they need it? If group 2 has that, maybe they don’t need Awesome Magical Powers to be a serious threat to the Monster of the Week.
…Which brings us to rule 5: A serious problem for one universe has to be a serious problem for the other.
This is in part a power-balance thing, in part a “don’t step on people’s schtick/awesome in deciding to be a hero in the first place.” After all, if guys from universe A can simply “set phasers on stun” and take down the Ultimate Villain from universe B, you have a suspension of disbelief problem. As in, “why didn’t somebody do this a long time ago?”
Not to mention you’ve just trashed every heroic act and struggle the heroes from B have accomplished, fighting the good fight against evil. Bad writer. Bad. No biscuit.
…Anyway. Just a few thoughts on what I think Makes Crosses Work. 🙂