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 some fanfictions, when Luffy meets his dad, there could be an emotional moment. Luffy might be surprised at first. His dad could give him some advice on his journey, like how to better face the powerful enemies in the New World. And maybe he'll share some secrets about Luffy's family history that he didn't know before.
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.
Luffy would probably be super excited. He's always been on this big adventure, and meeting his dad would be like a new milestone. He might jump up and down and start asking a million questions right away, like what his dad has been doing all this time and if he has any cool stories from his own adventures.
One common theme is the clash of different worlds. Since it's a crossover, Yusei Fudo from his own universe meets characters from another. This often leads to misunderstandings at first which then develop into alliances or rivalries.
Since 'lemon' usually implies a more mature or romantic theme, a common theme could be their romantic relationship. It might explore their first kiss, for example.
Another theme is adventure. Since they are in a world related to dueling, they often go on adventures together in the fanfictions. They face challenges during their dueling quests and grow closer as a result.