One possible plot could be that Luigi is kidnapped by a group of mischievous goombas who are being controlled by a powerful dark magic user. Luigi, while kidnapped, uses his wits to befriend some of the weaker goombas and gets them to help him send a message to Mario. Mario then has to figure out where Luigi is being held based on the clues in the message.
The kidnapper might be a deranged Toad. Maybe this Toad has been corrupted by some evil force and decides to kidnap Luigi for reasons only known to the twisted mind of this new 'evil Toad'.
Maybe it's a new villain in the fanfiction. This villain could be after Luigi because he thinks Luigi has some sort of special power or knowledge. So the villain kidnaps Luigi in a very sneaky way, like using a magic spell to put Luigi to sleep first and then whisking him away to a secret lair hidden deep in a dark forest.
Yoshi could also be the rescuer. Yoshi has his long tongue that can be used to grab things from a distance. He could use it to pull Luigi out of a locked cage or something. And Yoshi is very fast and can jump high too. So he'd be a great candidate to go on a rescue mission for Luigi.
Mario would definitely be a main character. Since Luigi is his brother, he would be the one most concerned about Luigi's transformation and would do everything he could to help him. Princess Peach could also be a main character. She might offer her castle as a safe place for baby Luigi and use her resources to find a solution. Toad could be there too, helping Mario with taking care of Luigi and providing useful information about the kingdom that could lead to reversing the spell.
A fanfiction that stands out is 'When Tails Met Luigi'. In this story, Tails accidentally lands in Luigi's world while testing a new invention. They start off a bit wary of each other but soon realize they can help one another. The story has great character development and a fun, engaging storyline that keeps you reading.
No. Mario and Luigi are typically seen as a friendly and loving brother duo in the Mario franchise. Depicting abuse between them in fanfiction is not in line with the positive values associated with the characters.
Another reason could be that in the official Mario games, Luigi is often seen as the secondary character to Mario. In fanfiction, making him cry can be a way to give him more depth and a chance to be the center of attention. It gives writers the opportunity to develop his character further and show his more emotional side, which fans might find really appealing.
I'm not entirely sure specifically what 'luigi snaps fanfiction' is about without more context. It could potentially be a darker take on Luigi's character where he snaps under some sort of pressure, like the constant threat from the villains in the Mushroom Kingdom. It might explore his psyche and how he deals with things differently than his more famous brother Mario.