Another plot could be that the mansion is actually a test set by an unknown entity. Luigi and Tails are chosen as the test subjects. They are faced with a series of challenges that test their friendship and abilities. For instance, they need to solve puzzles that require both Luigi's practical thinking and Tails' scientific knowledge. In the end, they pass the test and earn a reward that helps them in their future adventures.
Maybe the mansion is a place where time and space are distorted. Luigi and Tails get separated at first. Luigi has to find Tails while dealing with the strange things in the mansion like rooms that change shape. Tails, on the other hand, is trying to find a way to stabilize the time - space distortion. Eventually, they find each other and combine their efforts to fix the situation.
In this fanfiction, Luigi might be exploring the mansion when he stumbles upon Tails. Tails, being the tech - savvy character he is, could help Luigi figure out the strange mechanisms in the mansion. Maybe they'll find hidden passages together and encounter some spooky but friendly ghosts.
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.
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.
Mario could be a great addition. Since Luigi is Mario's brother, his presence could add some family - related drama or comical moments. Sonic could also be included. As Tails' friend, Sonic's brash and confident personality would contrast well with Luigi's more timid nature.
One possible plot could be that Tails is betrayed by a close friend who becomes jealous of his intelligence and skills. For example, Sonic might start to feel overshadowed by Tails and in a moment of weakness, turn against him. This could lead to Tails having to fend for himself and prove his worth all over again.
One possible plot could be that Tails is kidnapped by an old enemy seeking revenge. Maybe Eggman wants to use Tails' intelligence for his own evil schemes. He could hold Tails captive in his secret lair and try to force him to build some powerful weapon.
One possible plot could be that Tails is exposed to some dark energy or a powerful, corrupting artifact. This makes him gradually turn against his friends. He might start using his intelligence to outwit Sonic and the others for his new, evil goals.
Another plot could involve Tails trying to hide his sickness at first because he doesn't want to worry his friends. But as it gets worse, he can't hide it anymore. His friends then feel guilty for not noticing earlier and they all come together to take care of him and find a remedy.
One common theme is friendship. Luigi and Tails form a bond based on mutual understanding, despite coming from different universes. Another theme could be self - discovery. As Luigi comforts Tails, Tails may start to realize more about himself.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and you're likely to find some interesting 'Tails and Luigi' ones there.