How to write a crossover that holds
A crossover asks a reader to accept two sets of rules at once, and most of them fall apart at the seam rather than in the writing. The prose is fine; the worlds simply never agreed on anything.
Almost all of it comes down to four decisions you can make before chapter one — and making them late is why so many crossovers stall in the middle.
Decide whose rules win, before anything else
Pick one canon as the ground and let the other be the visitor. Whose physics, whose magic, whose scale of what is possible? Readers will follow almost any answer. What they cannot follow is a story where the answer changes depending on which scene you are in.
Write the ruling down as one sentence and keep it. "Their magic works here, our technology does not work there." That sentence will settle a hundred small questions later, and the questions it does not settle are the ones genuinely worth a scene.
The meeting needs a reason the reader believes
"They end up in the same place" is a premise, not a reason. The strongest crossovers give the collision a mechanism — a door that already existed in one canon, a shared enemy, an event big enough to pull on both worlds — and spend about a chapter making it cost something.
Decide early how much either side knows. A crossover where everyone takes the other world in stride loses the best scenes it had available: the confusion, the misreading, the moment somebody applies their own rules and is badly wrong.
Power levels are what actually kill crossovers
If one character can resolve the other canon’s central problem in a sentence, you no longer have a plot. This is the single most common crossover failure and it is usually visible from the outline, which is the cheapest place to fix it.
Three honest fixes: limit the power, raise the problem, or separate them. Cost, distance, ignorance or a rule from section one can all throttle an overpowered visitor. What does not work is everyone politely forgetting what they can do.
Write for the overlap, not for both fandoms
Your readership is people who know both, which is a fraction of either. That is the trade a crossover makes, and accepting it makes the writing much easier — you can skip the introductions and go straight at what only this combination can do.
Trying to serve single-fandom readers too is what produces the exposition that kills the pace. Pick the overlap, write for them properly, and let the tags do the filtering. The audience is smaller and considerably more enthusiastic.
Keeping two canons straight
Two casts, two sets of places, two rule systems, and every one of them can be contradicted by a scene you wrote three weeks ago. This is the practical reason crossovers get abandoned, and it gets worse exactly as the fic gets good.
In Bloomwright each character and place is a record — a role, what they want, what holds them back; what a place is and what it is known for — handed over on every reply, so a scene written in week six is written against the same facts as a scene from week one. The bonds across both casts are stored as directions rather than pairs, which is what you need when half your cast has only just met the other half.
Common questions
How do I decide which canon’s rules apply in a crossover?
Pick one as the ground and make the other the visitor, then write the ruling as a single sentence you keep — "their magic works here, our technology does not work there". Readers accept almost any answer. What breaks a crossover is the answer changing from scene to scene.
What do I do when one character is far more powerful than the other canon?
Limit the power, raise the problem, or keep them apart. Give the ability a cost, put it out of reach, make the character ignorant of when to use it, or raise the stakes until it is no longer decisive. What never works is everyone politely forgetting what that character can do.
Do readers need to know both fandoms?
Your actual readership is the overlap, which is smaller than either fandom and much more enthusiastic. Write for them: skip the introductions and go at what only this combination makes possible. Explaining both canons for single-fandom readers is what produces the exposition that kills the pace.