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.
One could be a story where Luigi loses a big Mario Kart race against his brother Mario and starts crying out of frustration. In this fanfiction, it might detail his inner thoughts about always being in Mario's shadow and how this loss feels like one too many.
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.
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.
Peach could be an important character. As a friend of Daisy and a princess in the Mushroom Kingdom, she could offer her wisdom on raising a child. Also, Yoshi could be there. Yoshi is always friendly and could be like a protector for the baby, watching over it when Luigi and Daisy are busy.
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.
There are likely some clean 'Luigi and Daisy' fanfictions out there. Some might depict them teaming up in a new Mario - related adventure, or just having a friendly conversation in the Mushroom Kingdom. These types of fanfictions are more in line with the family - friendly nature of the Mario franchise.
One common theme is the idea of redemption. Sometimes Bowser is shown on a path to change his ways, often with Luigi playing a role in that transformation. For example, in some fanfictions, Luigi's kindness makes Bowser question his evil actions.
Yes, there are some out there. You can try searching on fanfiction websites like Archive of Our Own. Just type in 'luigi x booette' in the search bar and you might find some interesting stories that fans have written.
One common theme is friendship. Writers often explore how Luigi and Peach can form a unique bond outside of the typical rescue - damsel in distress scenario. For example, they might be shown sharing a love for a particular type of music or food.