Maybe there's a dark wizard who has a vendetta against Natsu's family. When he finds out that Natsu is secretly a girl, he sees it as an opportunity to further his evil plans. He could try to kidnap her or use her as a bargaining chip against her family or friends. He might think that because she's a girl, she'll be easier to manipulate or capture compared to if she was thought to be male.
Perhaps a jealous rival. This person might be a male who is threatened by Ed's success or popularity. When they discover Ed is a girl, they could use this secret to try and drive her out of the group or community she's in. They might spread rumors or create problems for her.
Perhaps a powerful demon that has a grudge against Galuna Island. The demon sees the girl as a key to getting revenge on the island. Natsu, being the hero, would stand in the way. The demon could have minions that constantly try to capture the girl, creating a lot of action - packed scenes in the fanfiction.
Well, one possible plot twist could be that Natsu has been hiding her true gender to be able to participate in male - dominated activities like a certain guild or sport. Another could be that she's in love with a male character, but is afraid to reveal her gender in case it changes how he feels. Or perhaps she has a special power that is related to her being a girl secretly, like a unique form of magic that only female dragonslayers can possess.
Perhaps a rival adventurer who discovers Bilbo's secret and tries to use it against her. This antagonist could be jealous of Bilbo's place in the group and sees her secret as a way to undermine her and take her position. He might spread rumors or try to create situations where Bilbo's secret is likely to be revealed.
It could be a character who somehow discovers Izuku's secret and tries to use it against her. Maybe a rival at school who is jealous of Izuku's progress and sees this as a way to get her expelled or discredited in the hero community.
Yes, in some fanfictions Natsu can be secretly royalty. It adds an interesting twist to his character. Maybe he was a lost prince all along, and his dragon slayer powers are somehow related to his royal heritage. This concept can open up a whole new world of storylines, like him reclaiming his throne or having to deal with political intrigue in the magical world.
It could be another dark mage who sees the potential in baby Natsu and wants to use him for evil purposes. This mage might view Zeref as an obstacle in getting to Natsu.
It could be an old and powerful vampire lord who sees Natsu as a threat to his rule. Since Natsu is a new and different kind of vampire, the lord might want to eliminate him.
It could be a long - lost relative of an old enemy. For example, if there was a powerful dark wizard that Natsu defeated in the past, this relative might come seeking revenge and be powerful enough to kill Natsu.
Maybe an ancient dragon that has been awakened from its slumber and views Natsu as an intruder in the dragon world. Since Natsu is a new - made dragon, this old dragon might try to drive him out or even harm him.