Well, one possible plot could be that Naruto accidentally travels back in time and meets his mother. His mother, Kushina, might initially be shocked but then quickly recognize him as her son. She could then start to teach Naruto some of the Uzumaki clan's secret techniques that she never got the chance to pass on in the original story.
Another character could be the Nine - Tailed Fox. Kushina was once the jinchuriki of the Nine - Tailed Fox. When Naruto meets his mother, the topic of the Nine - Tailed Fox might come up. Maybe Kushina could tell Naruto some secrets about controlling the Nine - Tailed Fox's power that she learned during her time as its jinchuriki.
Goku's reaction could be a mix of shock and happiness. On one hand, he never expected to actually meet his parents. On the other hand, he has always longed for a family connection. He might start asking them a million questions right away about their lives, Saiyan culture, and what it was like when he was born.
She could be a former pirate captain herself. Maybe she was one of the most feared pirates in her time, and her skills and reputation were passed down to Luffy in some way, even if he didn't know it was from his mother.
His friends from the guild could be important. For example, Happy might be there to share in the joy of Natsu meeting his mother. Happy could also provide some comical relief during the emotional encounter.
One possible plot could be that Kakashi, while on a mission in a mysterious forest, stumbles upon a strange and powerful force. As he investigates, he is suddenly transported to a different realm where he meets his mother. His mother reveals secrets about his family's past and the origin of his Sharingan. Then they have an emotional conversation about how much she missed seeing him grow up.
The fanfiction might start with Natsu's search for his mother. When they meet, there could be some initial shock or disbelief. His mother could look at him with great love and pride. They might have a long conversation about his life so far, his friends, and his adventures. She could also introduce him to parts of his heritage that he was previously unaware of, which could shape his future actions and decisions in the story.
In this fanfiction, it could be that Luffy's mother was thought to be dead but was actually held captive by a powerful enemy. When Luffy finally meets her, she's weak but still full of spirit. She encourages Luffy to keep fighting against the corrupt world of pirates and marines alike. She might also have some inside information about the One Piece, like a clue that she's been holding onto all these years, which sets Luffy on a new and exciting part of his adventure.
A possible plot could be that Spyro discovers a hidden message or prophecy that indicates his mother is alive. He sets out on an adventure, facing dangerous terrains and foes. Meanwhile, his mother might be trapped in a different dimension. Spyro has to find a way to break through to that dimension. When they meet, his mother could be a very powerful dragon. She could teach Spyro about their dragon heritage and how to better control his powers for the good of their kind.
One possible reason could be a misunderstanding. Maybe some false information was spread about his mother, making Naruto believe she had abandoned him. Another reason could be related to the secrecy around his mother. If he grew up feeling alone and not knowing about her, he might develop resentment.