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.
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.
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.
There could be an original character created just for the fanfiction. A woman with a kind heart but a tough exterior, who was a great fighter. She could have met Luffy's father during a battle and they fell in love, and she became Luffy's mother.
Well, if the Luffy crew were to meet his mother in fanfiction, it could go in many ways. For one, she might be a strong pirate herself. She could then train the crew in new fighting techniques. Or perhaps she has connections all over the world of One Piece and could introduce the crew to important people. It would surely strengthen the bond within the crew as they learn more about Luffy's family.
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.
One idea could be that Luffy's mother was a powerful pirate herself. Maybe she was part of a crew that had a special mission, and she had to give up Luffy to protect him from the enemies she made at sea.
In Wesley's novel, Bai Su's mother finally found Bai Su, but at the end of the story, there was no mention of whether Bai Su had met his mother. Bai Su's mother was a very important character. She had experienced many twists and turns and pain, but in the end, she was relieved and happy.
There could be. Fanfiction is a place where all kinds of wild and creative ideas come to life. The idea of fem Harry being Luffy's mother is quite unique and would attract a lot of readers who are into crossovers. Some fanfic writers love to blend different universes together, and this would be a really interesting blend. But finding it might take some time. You should look for fanfics that deal with character relationships in an unconventional way and also check for any tags related to gender - bended characters from the Harry Potter series in the context of One Piece.