There's also a possibility that the main antagonist is another student at U.A. High School. This student could be competitive and feel threatened by the girl's relationship with Izuku. They might use underhanded tactics to try and make the girl fail or lose her confidence, adding a layer of drama and conflict to the fanfic.
Obviously, Izuku and All Might are the main characters. Izuku is the one with the relationship to All Might described in the title, and All Might is the object of Izuku's admiration as in the 'little girl' - like admiration.
Well, the plot could be about a young girl who idolizes Izuku. Maybe she's inspired by All Might's belief in Izuku and tries to follow in his footsteps. It could involve her training to be a hero, facing challenges like Izuku did, and learning valuable lessons along the way.
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.
Perhaps a jealous classmate who has always been in Izuku's shadow. Now that Izuku is a girl, this classmate sees an opportunity to undermine her. They spread rumors about how Izuku's transformation is some sort of cheat or that she's not the same reliable person she was as a boy. This classmate's actions and attempts to sabotage Izuku would make them the main antagonist.
Well, without reading it, we can only guess. It could be that Izuku, in the context of this fanfic, has a very cute and innocent admiration for All Might similar to how a little girl would look up to her hero. Maybe it shows Izuku following All Might around, learning from him, and getting into all kinds of situations where he has to prove himself while being under All Might's wing.
Maybe there's a villain group that targets Izuku specifically because of his money. They want to steal his wealth to fund their evil schemes. They might try to kidnap him or hack into his financial accounts. Izuku then has to use his wits as well as his money to outsmart them.
Maybe a mechanical tyrant in a steampunk - inspired setting. This tyrant has created an army of mechanical beasts and is trying to rule the city. Pyro, as a girl with fire powers, is the only one who can melt the metal parts of the beasts and stop the tyrant. The tyrant sees her as a threat to his rule and is constantly trying to eliminate her.
Maybe a malevolent scientist who has a grudge against female heroes. He believes that only men should have such powers and sets out to take the Omnitrix from Ben and use it for his own evil purposes. He underestimates Ben because she is a girl but soon realizes his mistake.
Obviously, All Might would be one of the main characters. He is the 'dad' figure in this fanfic. The girl could be an original character created just for the story. She could be a young girl with a unique quirk, like the ability to manipulate light, for example. Her name could be something like Lily.
Yes, there are quite a few out there. Some explore how All Might's relationship with Izuku would change if he was his father. For example, in some fanfics, All Might starts training Izuku much earlier in his life, which gives Izuku a huge advantage in developing his quirk. These fanfics often delve into the emotional side of their relationship as well, like how Izuku might feel having a hero as a father.