In such a fanfiction, Naruto might first be shocked to see his dad. He could learn a lot about his family's past directly from his father. Maybe his dad would teach him some new and powerful jutsu that was passed down in the family.
Obviously, Luffy and his dad are the most important characters. Luffy, the main character of the original story, is known for his carefree and determined nature. His dad, on the other hand, is a mysterious and powerful figure.
Well, in 'Luffy meets his dad fanfiction', it could be that Luffy's dad teaches him some new and powerful techniques. Maybe they team up to fight a common enemy. This would be a great opportunity for Luffy to bond with his father on a deeper level.
In such a fanfic, Naruto might finally get to understand his family heritage better. His dad could teach him some new jutsu that only the Uzumaki family knows.
Perhaps when Ash meets his dad, his dad is on a very important mission. Ash decides to join him, and during this journey, they face many challenges together. His dad could teach Ash some new battle strategies that are different from what he has learned before. They might also encounter some rare and powerful Pokémon that they have to work together to either capture or overcome. This meeting could be a turning point in Ash's Pokémon - training journey, making him more mature and experienced.
In a fanfiction plot, Yusei might meet his dad in a completely unexpected place, like in a different dimension. His dad has been trapped there for a long time. Yusei tries to find a way to get his dad out. Along the way, they share their stories, their hopes and dreams. Yusei realizes that his dad is not the person he thought he was, and his dad is also amazed by how Yusei has grown up. They finally manage to escape the dimension together and start a new life together.
When Yusei meets his dad in fanfiction, it could also be a bittersweet encounter. There might be some unresolved issues between them. His dad could be regretful for not being there for Yusei in the past. However, they could start to build a relationship from that moment on, slowly learning to understand and forgive each other.
They could develop a mentor - student relationship. His dad, being more experienced, could teach Ash advanced Pokémon training techniques. Ash, in turn, could show his dad his unique way of bonding with Pokémon, which his dad might not have seen before, thus deepening their connection.