For a Luigi's Mansion and Super Smash Bros. crossover fanfiction, Luigi could be in the middle of a Smash Bros. battle when a portal to his mansion opens up. Characters from the Smash Bros. roster get accidentally sucked in. Luigi has to lead them through his mansion, fighting off the more dangerous ghosts. Some characters like Kirby might try to eat the ghosts, while others like Link would use their weapons. Along the journey, they discover secrets about Luigi's Mansion that Luigi himself didn't know, all while trying to find a way back to the Smash Bros. arena.
One idea could be a Luigi crossover with Mario Kart characters in a story where Luigi has to compete in a special tournament to save the Mushroom Kingdom from a new threat that only targets the racers. Another could be a crossover with Zelda characters, where Luigi finds himself in Hyrule and has to team up with Link to defeat Ganon who has somehow invaded the Mario universe as well.
Begin by brainstorming the characters and settings you want to combine with Luigi. Let's say it's a crossover with the Final Fantasy series. Think about how Luigi would adapt to a world full of magic and complex characters. You could start with Luigi being magically transported to a Final Fantasy realm. Describe his initial confusion and then start introducing the main conflict. Maybe a powerful sorcerer in the Final Fantasy world is trying to take over the Mushroom Kingdom too, and Luigi has to find allies in this new world to stop him. Develop your characters' personalities and relationships as you write, and don't be afraid to let the story take unexpected turns.
Another option could be an evil version of Princess Peach. She could be possessed by some dark force. This would be a very interesting antagonist as Mario and Luigi would be hesitant to fight her at first because of their loyalty to the real Peach. But as she causes more and more chaos, they'd have to find a way to save her and the kingdom without harming her.
In a Luigi's Mansion and Pokémon crossover, a plot point could be that a rare and powerful Pokémon has been trapped in the mansion by an evil ghost trainer. Ash and his Pikachu arrive to help Luigi. Pikachu uses its electric attacks to stun the ghosts while Luigi captures them. They explore the mansion, looking for the Pokémon. Along the way, they encounter different types of Pokémon that have also been affected by the mansion's spooky aura. Some have become ghost - like themselves and Ash has to figure out how to help them while also finding the trapped Pokémon.
One idea could be that Mario and Luigi get transported to a strange new world where the rules of their own world are all mixed up. They have to use their combined skills in a whole new way to find their way back home. For example, Luigi might have to be more brave and take the lead in some situations while Mario has to use his speed in different ways than he's used to.
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.