Sir Eam wasn't Luffy's mother. Oda once said that his mother was not part of One Piece's story and would not appear. Luffy's mother did not belong to this story. From the logic of the story, if Sir Eam was Luffy's mother, it didn't make sense for him to kill Luffy. Moreover, judging from Eam's gaze in the animation, the possibility of Eam being a man was extremely high. So, Sir Eam wasn't Luffy's mother.
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.
In One Piece, Luffy's mother was not explicitly mentioned. Oda had once said that mothers were the natural enemy of adventure. Luffy began his adventure after leaving his mother's embrace, so Luffy's mother was not part of the story he told and would not appear. However, there were many speculations among the fans. For example, some people guessed that it was Seven Martial Haisha Krokdal. The reason was that Old Sha looked like a girl in a photo when she was young. Ivankov could change people's gender and said that Old Sha had something on him. They speculated that Old Sha used to be a princess of a fallen country. After being saved by the dragon, she fell in love and gave birth to Luffy. Later, Ivankov turned her into a man. There were also people who guessed that it was Sir Eam, because Sir Eam was holding Luffy's bounty and had the same huge straw hat as Luffy in front of him. However, after Luffy's voice actor, Makoto Tanaka, asked Oda for confirmation, this speculation was denied. There were also people who guessed that it was Luffy's hometown, the owner of the Windmill Village bar, Makino. However, Oda's drawing of Makino holding a child overturned this speculation. There were also people who thought that it was the red-haired sisters, but there was no conclusive evidence.
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.
Sir Eam was the behind-the-scenes ruler of the world government in One Piece. There were many speculations and inferences about his background. One view was that Sir Eam was most likely the final victor of the power struggle between the 20 kings when the World Government was established. He used some means to replace the 20 kings and took the huge straw hat that symbolized the highest power for himself. There were also rumors that Sir Eam was suspected to be one of the kings of the first 20 people 800 years ago, Saint Eam of the Narona family. Sir Eam might also have a close relationship with Alabastan. Queen Lily of the Kingdom of Alabastan was one of the first 20 kings. She refused to disappear after Mary Joshua became a Celestial Dragon. Cobra said that there was no record of her returning to Alabastan in the historical records. Alabastan was succeeded by her younger brother, and there was a letter written by the Queen of Alabastan that mentioned "D's Will." Lady Eam was a woman. She had once held a photo of Princess Vivian and walked to the Void Throne in front of Cobra while muttering "Lily." These details made people speculate that Lady Eam's true identity might be Queen Lily. There was also the possibility that Sir Eam could use the fruit of the surgery to perform a personality transplant or other methods to possess Queen Lily's body and obtain eternal life. During the World Conference, Eam went to the secret room deep in Pangu City where the huge straw hats were kept. Then, he went to the flower garden and cut Luffy and Blackbeard's bounty into pieces, while admiring the photos of White Star and Vivian. When Nefert D. Kobra of the Kingdom of Arabastan came to ask about Queen Lily and the meaning of "D", Eam appeared and said that "D" was once their enemy. Due to the fault of "Lily", the fragments of history were spread all over the world. He asked Kobra to reveal the name left in Lily's letter. After Kobra answered, Eam attacked and pierced her body. Sabot rushed out to attack. Eam used his ability to neutralize the flames and chased after Sabot and Kebra with the Five Old Stars. Kebra protected Sabot and escaped, but he died of serious injuries. After the World Conference ended, the revolution of the Eight Kingdoms, which was influenced by the Revolutionary Army, broke out. Em ordered the Five Old Stars to use the weapons developed by Bergeroponk to wipe out the Kingdom of Lucia. Eam was also afraid of the D-race and decided to clean up the world. He had tried to assassinate Vivian and Cobra in Arabastan but failed. In addition, Sir Eam was extremely powerful. With just a single long-range attack, he was able to finish off Saten Saint, who Luffy had tried everything he could to.
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.
From some of the information, some of the settings involved becoming Eam's daughter. For example, there was a story that said," Miss Youna, your current identity is Eam's daughter." Also," I, Yi Manzi, want you to hit me! The prerequisite is that you can defeat a general. Oh right, I'm Eam's daughter!" Such a plot setting, but there was no more detailed description of the story or the related effects after becoming Eam's daughter.
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.
As of now, the true identity of Luffy's mother had not been clearly revealed in the original work of One Piece. Although there were many speculations, such as some people believed that Luffy's mother was a Heavenly Dragon, and speculated that the dragon and the female Heavenly Dragon loved each other and gave birth to Luffy, but were forced to separate due to the rules of the Heavenly Dragon, which led to the dragon's hatred of the rule of the Heavenly Dragon and the establishment of the Revolutionary Army. There were also speculations that his mother was a Sandia, and the reason was that the tattoo on the dragon's face might be related to the tattooing custom of the Sandia. There were also some strange speculations, such as that she was Shanks's sister, but these were just fans 'speculations and not the truth officially announced.