They would probably be very surprised at first. After all, they might not have expected to ever meet Luffy's mother. Then they would be excited to learn more about Luffy's past from her.
Well, his parents might be a bit shocked at first. I mean, Luffy's crew is full of such unique characters. They could start by testing the crew's loyalty to Luffy. And then, they might offer some valuable advice or even some special training. Like, if his father has some unique skills, he could pass them on to the crew to make them even stronger for their future adventures.
It could be an old and powerful wizard who has been in hiding for years. Maybe he was a friend of Dumbledore's but had some differences with him, so he stayed away from the main events in Harry's life until now.
There were a few novels that could be recommended, including How to Stop Death, The Idiot Life from the End of the Marriage, and The Strongest Nanny Dad in the City. The protagonists in these novels lived with their grandfather, and the plots were different. " How to Stop Death " was a fantasy novel about swords and magic. It told the story of a young man who was destined to find his own fate in the process of pursuing justice. " The Idiot's Life from the End of the Marriage " was a light-hearted and funny urban novel. The main character was the second-generation heir, Wang Zhihao. He lived in the countryside with his grandfather, and a series of interesting stories happened. " The City's Strongest Babysitter " was a city-city supernatural novel. The main character was the leader of the Dragon Sect who had become a useless nanny. He and his grandfather had experienced many challenges together. These novels could be used as recommendations for the protagonist and grandfather's life.
The book 'Battle Break's Strongest Villain' met the requirements. The main character, Hun Tianchen, was the young clan leader of the Hun clan. His father was Hun Tiandi. I recommend this book to you, I hope you like this fairy's recommendation, muah ~šš
In this fanfiction, the meeting with the grandfather could also be a source of conflict. Perhaps the grandfather has certain expectations or traditions that seem to be at odds with Shikaneji's relationship. This would create drama as they try to navigate their feelings while also dealing with the grandfather's views.
It was quite common for the old chief to be his grandfather in novels about rebirth because the background of the characters and the historical background were often reconstructed. In this case, some novels might set the old chief as grandpa because grandpa was a very common title in traditional Chinese culture to express respect and care for the elders. At the same time, maternal grandfathers were usually regarded as important figures in the family with a certain amount of power and status.