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.
Their relationship could develop through shared interests. For example, if they both have an interest in exploring the more mysterious parts of the Mushroom Kingdom. They could start going on adventures together, and as they face challenges like tricky terrains or new enemies, they rely on each other and grow closer.
In 'War' by Luigi Pirandello, one of the key aspects is the exploration of human nature during times of conflict. The story shows how different people react to the idea of war. Some may be filled with patriotic fervor, while others are more hesitant or even opposed. It delves into the complex emotions that war stirs up, like fear, pride, and a sense of duty.
The relationship could start with Luigi being a shy admirer of Peach. He might do small things for her like bringing her special flowers from the Mushroom Kingdom. Peach starts to notice Luigi's thoughtfulness and begins to seek his company more often. As they spend more time together, they discover they have a lot in common, like their love for the peaceful parts of the kingdom. This mutual understanding leads to deeper feelings and eventually their decision to get married.
Princess Peach would be a great addition. She could be in trouble as usual, and Mario and Luigi have to rescue her. But this time, she could also play an important role in helping them solve a mystery or defeat a powerful enemy. For example, she might have some knowledge about an ancient artifact that the brothers need.