A few things I've noticed about fanfiction tropes, why they keep showing up, and what they actually do when they're working.
Enemies-to-lovers gets treated like the flagship fanfiction trope, and maybe it is, but the interesting part isn't the enemies or the lovers. It's the middle. The part where two people who've built a whole identity around opposing each other have to quietly admit they were wrong. That's not a romance beat. That's a character-development engine. Most published novels skip it or rush through it because the plot needs to move. Fic writers sit in it for forty thousand words, and their readers love every page.
The coffee shop AU gets dismissed as fluffy. But stripping away every external source of tension and asking a relationship to survive on pure dialogue and interiority is one of the hardest things you can write. There's nothing to hide behind. No dark lord, no apocalypse, no ticking clock. Just two people and a counter.
E.L. James started Fifty Shades of Grey as a Twilight fanfic called "Master of the Universe." The characters were Edward and Bella. The setting was different, the power dynamics were cranked up, the tone was unrecognizable. But the emotional core, a reserved, controlling figure and an ordinary woman drawn into his world, was the same trope at work. When she filed off the serial numbers and published it, what survived wasn't the Twilight DNA. It was the trope. The trope was strong enough to carry a book on its own, and then a franchise, and then a cultural moment that most literary novelists would quietly envy if they were being honest about sales figures.
Hurt/comfort is probably the most misunderstood trope outside of fandom. From the outside it looks like characters getting injured so someone can tend to their wounds. From the inside, it's about permission. One character is hurt, which means the other character now has a socially acceptable reason to show tenderness, to touch, to be vulnerable. The injury is a door. You walk through it into the emotional scene you actually wanted to write.
There was only one bed. I know. I know. But the reason it keeps working, decade after decade, is because it externalizes an internal problem. Two characters who won't admit they want to be close are physically forced into proximity by a circumstance neither of them chose. The trope does the work of ten pages of internal monologue in a single logistical detail.
Rainbow Rowell wrote in Fangirl: "The whole point of fanfiction is that you get to play inside somebody else's universe. Rewrite the rules. Or bend them. The story doesn't have to end." That last line is the one that sticks. Because most tropes in published fiction resolve. In fic, they recur. The same trope gets written a thousand different ways, and each version teaches you something about what the trope is actually doing underneath the surface.
I'm still not sure whether the soulmate AU works because readers want fate or because they want the characters to have permission to feel something they'd otherwise talk themselves out of. Those are very different desires. The first one is romantic. The second one is almost therapeutic. And I think different readers come to the same fic looking for different things, and the trope is elastic enough to hold both.
The 5+1 format (five times something happened, plus one time it didn't) is a compression tool that published fiction rarely uses but probably should. You can cover years of a relationship in three thousand words. Each section carries its own weight. The reader fills in the gaps between them. It trusts the audience in a way that most novel structures don't.
Ali Hazelwood's debut The Love Hypothesis started as Reylo fic, a Star Wars pairing between Rey and Kylo Ren. She was in STEM fandom spaces, writing about intense, prickly dynamics between brilliant people who couldn't get out of their own way. When she moved to original fiction, the fandom voice came with her. The banter, the emotional precision, the specific way her characters deflect with humor before collapsing into sincerity. Her readers from fandom recognized it immediately. New readers just thought it felt unusually alive.