You can develop Wario by showing his softer side. Maybe he has a hidden love for cute things like small Toads. For Waluigi, explore his insecurities. He's often seen as the sidekick, so show how he wants to prove himself. They can develop together by having them share their dreams and fears with each other during a long journey.
The origin of Wario dates back to his creation as an antagonist to Mario. He was initially designed as a greedy and more villainous version of Mario. Waluigi, on the other hand, was created later as a partner in crime for Wario. He was designed to be lanky and purple, sort of an 'evil Luigi'. He was introduced to be a counterpart to Luigi in the same way Wario is to Mario.
The story behind Wario and Waluigi is somewhat mysterious. They were introduced to add more diversity and conflict to the Mario franchise. Wario was initially Mario's rival, and Waluigi emerged later to partner with him. Their personalities and motives are mainly shown in their actions within the games.
Sure. Wario was made as a sort of anti - Mario, being greedy and bad - tempered. Waluigi was created later as Wario's partner, a lanky and sneaky character to oppose Luigi.
Toad could be a character in the fanfiction. He could be the messenger or the one who sets up the initial situation that brings Wario and Waluigi together. Captain Toad could also be included. His treasure - hunting skills could be useful in a story where Wario and Waluigi are on a quest for something valuable. And Rosalina could make an appearance, perhaps as a wise figure who gives advice to Wario and Waluigi during their adventures or disputes.
A possible plot for a 'Waluigi x Wario' fanfic could be that they both get transported to an alternate dimension. In this strange new world, they have to rely on each other to find their way back home. Along the way, they encounter various challenges that force them to combine their unique skills. For example, Wario's brute strength and Waluigi's lanky agility could be used to overcome obstacles like giant monsters or tricky mazes. This adventure could also lead to a deeper understanding of their relationship, perhaps even a friendship that was previously unacknowledged.
A plot could revolve around them starting their own business together. They face all sorts of challenges like dealing with difficult customers and financial problems. Along the way, they learn more about each other and their friendship deepens. It could be a comical take on the struggles of entrepreneurship. Or perhaps they are framed for a crime they didn't commit and have to work together to clear their names. They'd have to use their wits and their knowledge of the Mushroom Kingdom to find the real culprit.
One popular 'waluigi x luigi fanfiction' is 'The Unlikely Bond'. It explores an alternate universe where the two characters have to team up against a common enemy. It's full of interesting character interactions and some really cool action scenes.
Often, there's a theme of redemption in these fanfictions. Waluigi, who is usually portrayed as a bit of a villain, may find redemption through his relationship with Luigi. Also, the theme of self - discovery is quite prevalent. As they interact, both characters learn more about themselves and their place in the Mario universe. There can also be a theme of overcoming differences, as they are very different characters in terms of appearance, behavior, and motives.
Another theme is friendship. Despite their differences, the fanfictions usually explore how they can form a bond. This could be because they are both related to Mario in different ways, and they find common ground in that family connection. They learn to accept each other's quirks and work together.