Often, in 'izuku super soldier fanfiction', there are villains from the original series that become more menacing when facing Izuku as a super soldier. For example, All For One might be an even greater threat as he tries to either control or destroy Izuku.
Some fanfictions introduce new antagonists specifically designed to challenge Izuku's super soldier abilities. These could be other super - powered individuals with a vendetta against the super soldier program or Izuku himself. They might have abilities that directly counter Izuku's, like a villain with the power to nullify his enhanced strength.
In certain 'izuku super soldier fanfiction', the main antagonist could be a corrupt organization within the hero society. They see Izuku as a threat to their power and influence because of his super soldier capabilities. So they try to undermine him and his allies at every turn, creating a complex web of conflict.
It could be the traditional God of War villains like the gods who oppose Izuku. Maybe Ares, if he's still around in the fanfiction's version of the God of War world, would see Izuku as a threat and try to stop him.
Some of the teachers at U.A. might also be antagonists. They could be secretly working for an organization that wants to control Izuku's powers for their own evil purposes. They would pretend to be helpful while actually setting traps and spreading misinformation about him.
Perhaps a new villain could emerge who specifically targets Izuku because of her new gender identity. This villain might see her as a symbol of change in the hero society that they don't want to happen. So they constantly attack her and try to break her spirit, thinking that if they can stop her, they can halt the progress of female heroes in general.
In Kidnaped Izuku Fanfiction, likely antagonists could be rival villains who see Izuku as a threat to their plans. For example, those who want to stop him from becoming a great hero. It could also be villains who are after his unique quirk. Maybe they want to study it or use it for their own evil purposes. Another possibility is that it could be a group with a vendetta against the hero school he attends, and they target Izuku to send a message.
If Izuku is an alien, the antagonists could be other aliens who want to use his unique powers for their own evil purposes. Maybe they see Izuku as a key to conquering Earth or other planets.
There's a fanfiction where Izuku as a super soldier has to train a group of young heroes. In this 'izuku super soldier fanfiction', he uses his military - like discipline and combat skills to bring out the best in the trainees. It's really interesting to see how he imparts his knowledge and experience.
It could be other villains who are jealous of Izuku's powerful dragon quirk. They might see his quirk as a threat to their own plans for domination or chaos.
The League of Villains would be obvious antagonists. They'd see Izuku with the All for One quirk as a threat or a potential recruit. Shigaraki, in particular, might be envious and want to take the quirk for himself.
Maybe there are some anti - hero groups who target Izuku because they see him as a threat to their plans. They could try to kidnap him or steal his research on heroics from the university.
Maybe a powerful Titan who has somehow managed to get hold of the super soldier technology. They could use it to create an army of super - powered minions and try to take over Olympus again. This would force Percy and his friends to not only fight the Titan but also the super - powered forces.