Write the best fanfic ever about Harry Potter, and most people still will never read it because your audience is restricted to Harry Potter fans who read fanfiction - a doubly restricted group.
Woah, I never though of it like that before.
We should be writing crossovers!
We should be writing crossovers!
I thought crossovers only appeal to fans of both works, and hence that works the opposite of the way you thought it would?
"The kind of classic fifties-era first-contact story that Jonathan Swift might have written, if Jonathan Swift had had a background in game theory."
-- (Hugo nominee) Peter Watts, "In Praise of Baby-Eating"
Three Worlds Collide is a story I wrote to illustrate some points on naturalistic metaethics and diverse other issues of rational conduct. It grew, as such things do, into a small novella. On publication, it proved widely popular and widely criticized. Be warned that the story, as it wrote itself, ended up containing some profanity and PG-13 content.
PDF version here.