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.
Lex Luthor could be a great main antagonist. He might see Izuku as a threat to his plans, especially if Izuku's unique abilities and sense of justice start to disrupt Luthor's schemes in the combined universe. Luthor could try to manipulate events to turn the Justice League against Izuku or find a way to steal Izuku's power for himself.
Maybe it's about Izuku using his wealth to support his hero training in unique ways. He could afford the most advanced support items and training facilities.
A powerful sorcerer who wants to steal Izuku's Unlimited Blade Works power for his own evil purposes. He could be from a hidden world of magic and have minions that Izuku has to fight through.
It could be Aizawa himself if he's being controlled by something or has some sort of ulterior motive that is not in Izuku's best interest. But if not Aizawa, then there could be a hidden villain who is manipulating Aizawa to kidnap Izuku.
Another option is that there is a group in his own world that doesn't like the idea of someone coming back from another world. They might see Izuku as an anomaly and try to capture or study him. This group could be a secret organization that has its own motives, and in their attempts to deal with Izuku, they end up being the main antagonists.
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.
There could be an ancient evil entity that has been awakened by Godzilla's presence. This entity wants to destroy everything in its path and sees Godzilla and Izuku as obstacles. It has the power to corrupt other creatures and turn them against our heroes, making it a very formidable antagonist.
Maybe it's Obadiah Stane. He's jealous of Iron Man's success and with Izuku in the picture, he sees an opportunity to get rid of Iron Man once and for all. He could also see Izuku's quirk as something he can use to his advantage, so he tries to capture Izuku and use his power against Iron Man.
Another possibility is that in the world where Goku Black comes from, there is an enemy who follows Goku Black to Izuku's world. This enemy could be a powerful Saiyan or a being with a dark ki similar to Goku Black. They might see Izuku as a weakness in Goku Black's plan and try to eliminate him first, making them the main antagonist in the fanfiction.