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.
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.
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.
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.
I'm not sure if there is specifically a 'bowser x luigi mpreg fanfiction' available. Fanfiction can cover a vast range of themes and pairings, but this particular one might be a bit of a niche or unusual concept. You could try searching on some major fanfiction websites like Archive of Our Own or FanFiction.net to see if it exists.
Typically, Mario would be the hero in a 'bowser kidnaps peach fanfiction'. He has a long - standing relationship with Peach and is always ready to rescue her from Bowser.
Well, in this kind of fanfiction, perhaps Bowser kidnaps Peach because he wants to make her his queen. Peach, being the strong - willed character she is, tries to escape on her own while Mario is on his way. Bowser sets up all sorts of traps to keep Mario from reaching Peach. The fanfiction could also explore the relationship between Bowser and his minions during this time, like how they take care of Peach while she's captive.
One interesting plot could be that Luigi and Bowser are forced to team up against a common enemy greater than both of them. This could lead to a lot of comical moments as they have very different personalities. Luigi is often timid and Bowser is aggressive.
You can try websites like FanFiction.net. It has a large collection of fan - made stories, including ones about Bowser and Luigi. Just search for their names in the search bar, and you'll get a list of relevant fanfictions.