Since I don't read fanfic that much outside of certain contexts, I actually hadn't heard of WBWL before, so it was new to me. But anyway, when your post then introduced me to the concept, I thought to myself: "A prophetic hero whose twin is mistaken for the prophetic hero? That sounds like something I'd write!" Except I'd obviously write it into an original world, and make the one who's not the hero the protagonist of the story, et cetera you'll see how I interpreted the general premise below.

So I came up with this activity that's kind of a cross between Round Super Robin (hS, BRING THAT BACK!!!) and Fill the Plothole, based around the idea of taking a trope from fiction or fanfiction and actually writing it well, with or even without the transport to an original story. What one does is take one or two tropes from the game's list of tropes (which I shall start below) and write a fanfic or story premise stemming from a skillful reinterpretation of it, then come up with a new trope to replace the old one in the list, and put it at the end of the post. Those who feel inspired by a particular premise another person made can write one-shots or scenes as they please.

So, here's an example where the result is original fiction:

Trope: WBWL
Title: The World Belongs to Amalie
Premise (pitched like on the back of a book):
They've got the wrong girl!
Kate and her twin sister Amalie are two girls growing up in a world where such things as warriors and sorcerers roamed long ago. On the last day of summer, Kate receives a surprising message: she has to transfer to a special academy because she is the heroine of an ancient prophecy. The only problem is, Kate knows she's not the real heroine - it's Amalie, who Kate has seen fulfill the first part of the prophecy by developing magic powers. Kate would do anything for her sister, and it's killing her to see how unhappy Amalie is at having her special destiny unrecognized. But it seems like no one can be convinced that quiet Amalie could possibly be fated for greatness, and Kate fears that it might send her down a dark path...
Other Info: The part of the prophecy that had to do with which of the twins was the heroine was mangled in translation - maybe one has a feature that the other doesn't and the prophecy originally said the twin who doesn't have it but the translation says the twin that does, or vice versa. Also, in case it's not clear from the description, this isn't so much of an urban fantasy setting as it is a post-high fantasy kind of world, if that makes any sense - it's the result of what happens when a high fantasy society undergoes all the societal development that our world has.

And here's an example of one where the result is fanfiction, in this case Creepypasta fanfiction:

Trope: OC x Jeff
Title: Chasing Jeff the Killer
Premise:
One had a crush on him in high school, but was never able to confess before it was too late. Now a criminal psychologist, she’s determined to fix him. The other is his partner in crime, and just as determined to get into his pants. Ten years after Jeff started killing, two women are fighting to decide his fate.
Other Info: I wish I had a scene prepared for this. If I did, it would be NSFW, because my really disheveled adult Jeff and his OC partner-in-crime-then-mistress both curse like sailors. The psychologist doesn't "win" the fight for Jeff, but she still gets a happy ending: Jeff and his mistress are imprisoned, and the psychologist realizes that he's just not worth it anymore.

Now here's the list of tropes we'll begin with!
-The protagonist, a teenage girl, learns she's not human. (YA novels)
-The protagonist's love/hand/body is sought after by a significant number of people. (Fanfic and, of course, the entire genre of harem anime)
-The story has an ensemble cast, and every protagonist has their own antagonist to fight. (Not sure if it's a trope, but it was one of the more retrospectively humorous things about the RP group I was in some years ago.)
-The story's title is "[Big Bad's name here]'s [something]." (Er, historical fiction and nonfiction... let's leave it at that. The tricky bit about this one is that for it to work, people have to know that the name they're looking at is that of the Big Bad. Which is part of why you don't see it anywhere else!)
-Love is a literal power. (Stories aimed at kids and teens)
-The protagonists, while they're not going saving the world and possibly going to school, have a band or idol group. (Magical girls in both anime and Western cartoons)

So, let the game begin! Have fun, everyone!
-Twistey