It could be an amateur writer who is a huge fan of One Piece and really likes Luffy. They might have come up with this idea to explore a different side of Luffy that isn't shown much in the original series.
Well, it could be about Luffy having a secret power that no one in the Straw Hat crew knows about. Maybe it's a power that he got from a mysterious encounter and he's keeping it a secret to protect his crew from potential danger.
Rayleigh would be a great candidate. He's an experienced pirate and has already trained Luffy for a while. He has a lot of knowledge about the world of pirates and could give Luffy a talk about the deeper secrets of the Grand Line or how to deal with the powerful enemies that lie ahead.
It could be Hinata. In the original series, Hinata has a deep affection for Naruto. In a fanfiction, it would be a natural progression for her to be his secret fiance. Maybe they got engaged secretly to avoid some political unrest in the village.
It might also be Gaara. Their initial encounters were filled with tension, but as the story could be twisted in fanfiction, that could develop into a different kind of relationship. Gaara's own journey of self - discovery and Naruto's influence on him could be a basis for a romantic relationship in fanfic.
The author could be a relatively new writer in the fanfiction world who is trying to make a name for themselves with this creative concept. Maybe they were inspired by other Naruto fanfictions or by the original Naruto series itself. They might be someone who has a deep love for the Naruto universe and wanted to add their own unique twist to it by giving Naruto 12 wings.
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.
It could be Misty. They have a long - standing relationship in the Pokemon series. They've traveled together and shared many adventures, so in a fanfiction, it's not a stretch to imagine her as a secret girlfriend.
It could be an original character created just for this fanfiction. Maybe a girl who Stiles met during a summer vacation and had a brief fling with.
The father might be a character who is in a position of power and Sakura is afraid of the consequences if his identity is revealed. For example, he could be a nobleman or a powerful wizard in a magical world. Since having a relationship with him might be frowned upon or dangerous, she keeps the baby a secret.
It could be Ginny. Even though in the original story they are married and have kids in a more normal way, in fanfiction, there could be a twist where their relationship had a secret earlier on and she is the mother of the secret child.