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.
I'm not sure specifically as it could be a very fan - created and unique story. But generally, it might be a fan - made fictional work where Bowser, the villain from the Mario universe, kills Luigi. It could be a dark take on the usual Mario - related stories, perhaps exploring different themes like power, loss, or the unexpectedness of a major character's death in a fan - created scenario.
Yoshi could potentially be included as a popular character in these fanfictions. He could be Luigi's sidekick when he goes to meet Bowser. Yoshi's cute and helpful nature could add an interesting dynamic, either by making Bowser less hostile or by getting into some sort of trouble that Luigi and Bowser then have to solve together.
The 'Mario and Luigi Inside Bowser Story' could be centered around the concept of exploring Bowser's insides as an uncharted territory. Mario and Luigi, being the heroes, would be facing challenges such as Bowser's internal defense mechanisms. They might need to use their unique skills like Mario's jumping and Luigi's high - jumps in new and creative ways to progress. Also, they could be looking for a weakness within Bowser that can only be found from the inside. It could also involve some comical moments as they interact with the strange things inside Bowser.
In this kind of fanfiction, perhaps Luigi would be really scared at first. Bowser might use Luigi as a bargaining chip against Mario. Luigi could try to find a way to escape from Bowser's lair on his own, or he might be waiting for Mario to come and rescue him.
There might be a fanfiction where Luigi gets lost in Bowser's castle and Bowser, instead of capturing him as usual, shows him around the castle out of curiosity. It's a unique take on their relationship. They end up sharing some secrets about their respective kingdoms.
Another theme could be adventure. They might go on some sort of journey together, either in the Mushroom Kingdom or in some new, made - up world. It's also possible that some fanfics have a theme of redemption, where Bowser has a change of heart through his interactions with Luigi.