Maybe a new character could be the main antagonist. A powerful and mysterious being that Luigi stumbles upon during his escape. This character might have some vendetta against the Mario brothers or just be generally evil and wants to stop Luigi's journey for its own malicious reasons.
Another possibility is that the main antagonist is a corrupted version of one of Luigi's friends. Maybe a spell was cast on them, and they now try to stop Luigi from achieving his goals while he's running away. This would add an interesting layer of conflict as Luigi has to face someone he once trusted.
Well, perhaps Luigi runs away because he's tired of living in Mario's shadow. He might decide to explore new lands on his own, meet new characters, and have adventures that are just about him, not always being the sidekick.
Maybe an organization that specializes in using the Sharrigan for malicious purposes. They don't want Naruto to expose their secrets, so they send their Sharrigan - users to hunt him down. This organization could be made up of power - hungry individuals who will stop at nothing to achieve their goals.
Bowser could be the main antagonist. He might see Luigi's transformation as an opportunity to mess with Mario and the Mushroom Kingdom. He could try to use Luigi's new form to his advantage, perhaps by trying to make Luigi a spy or by using his new identity to cause chaos among the kingdom's residents.
Another option could be an evil version of Princess Peach. She could be possessed by some dark force. This would be a very interesting antagonist as Mario and Luigi would be hesitant to fight her at first because of their loyalty to the real Peach. But as she causes more and more chaos, they'd have to find a way to save her and the kingdom without harming her.
Wario could also serve as an antagonist. He might be envious of Luigi's new married life and try to steal their treasures or disrupt their happiness. Maybe he spreads false rumors about Luigi in the Mushroom Kingdom just to create chaos in their lives.
Obviously, Percy would be the main character. But there could also be his friends who might be trying to find him after he runs away. For example, Annabeth could be a major character in the story as she would be worried about Percy and use her intelligence to track him down.
In some fanfictions, the character's internal conflict can be a strong motivation for running away. Twilight might be struggling with her own identity or self - worth. She is a very studious and responsible pony, but she could be having doubts about whether she's truly making a difference or if she's just living up to others' expectations. Running away could be her way of trying to find herself without the pressures of her normal life in Ponyville.
One possible interpretation could be that the female character named Hiccup has some sort of internal turmoil or external pressure that makes her run away. In fanfiction, this could be a setup for an adventure or self - discovery journey. Maybe she's running away from a restrictive society or a difficult family situation.
Well, if Mal runs away in 'descendants' fanfiction, she might be seeking freedom from the expectations placed on her. Maybe she's tired of being in the middle of all the drama and power struggles in Auradon. She could go to a hidden place, perhaps an enchanted forest where she can be alone and figure out who she really is without all the royal and villainous influences.
The fanfiction about Ruby running away might focus on her internal conflict. Ruby is often seen as a strong leader in RWBY, but running away shows a different side. It could be that she's running from her responsibilities, feeling overwhelmed. Or perhaps she's running towards something she believes in but others don't understand. This could lead to a story of self - discovery and growth as she figures out what she really wants and how to get back on track.