The main characters are Arlong and Luffy. Arlong is known for his tyranny over Nami's village and his general dislike for humans. Luffy, as the captain of the Straw Hat Pirates, is all about making friends and having adventures. In this fanfiction, their personalities will clash and blend in unique ways. It's likely that the story will explore how their different natures interact and how they might change as a result of their encounters with each other.
One possible plotline could be Arlong and Luffy having to team up against a common enemy in a very unlikely alliance. This would force them to put aside their differences and learn to work together.
Luffy is still alive and kicking in the manga. The story keeps evolving with him at the center of the action, encountering new enemies and growing stronger.
Nami would be a great character to include as she has a deep - rooted history with Arlong. Their interactions can add a lot of drama and emotion to the story. Luffy is also a good choice as he is the main protagonist and his values of freedom and justice would contrast sharply with Arlong's tyranny.
One idea could be exploring Arlong's past before he became so cruel. Maybe there was an event that hardened his heart. Another could be a fanfic where Arlong has a change of heart and tries to make amends for his actions. For example, he could start helping the very people he once oppressed.