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.
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.
One possible plot is that Naruto's dad appears to him in a dream world at first. They start to talk, and his dad reveals some secrets about Naruto's birthright.
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.
One possible reason could be that Naruto's dad was not around during his childhood. In the Naruto story, Naruto grew up alone and ostracized, and he might feel that his dad abandoned him, which could lead to hatred in a 'dark Naruto' scenario.
There are also many fanfics that explore positive relationships in the Naruto world. Some might focus on Naruto's family in a more positive light, such as how Minato and Kushina's love led to Naruto's birth and how they would have wanted to see him grow up in a world of peace and prosperity, rather than the inappropriate and unethical ideas presented in that kind of fanfic.
In the 'naruto meets his brother fanfic', it's possible that the brother has a different view of the ninja world compared to Naruto. This could lead to some interesting debates between them. The brother might be more cynical or have a darker past that Naruto has to come to terms with. There could also be a rivalry at first, but as they get to know each other better, they form a strong bond. Maybe the brother has some special abilities that Naruto has never seen before, and Naruto is eager to master those abilities as well.