Perhaps some of the more powerful mythical beasts in the God of War world could serve as main antagonists. They are often depicted as being extremely dangerous and difficult to defeat. Izuku would have to use all of his skills and intelligence to overcome them. And if the fanfiction takes a darker turn, it could be that Izuku is pitted against some of his own friends or allies who have been corrupted by the dark forces of the God of War world, which would make for a really tragic and intense 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.
There could be an ancient evil force that awakens because of Luffy's new powers. This force was once sealed away but now wants to destroy Luffy and everything he stands for, seeing him as a disruption to the balance it once knew.
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 God of War 1, the main antagonist is Ares. In God of War 2 and 3, Zeus is the main antagonist as he betrays Kratos and tries to stop him from getting revenge.
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.
Perhaps there are demons or dark forces that are trying to disrupt the union between Bella and the God of War. These antagonists could be bent on causing chaos and destruction in their world, seeing their relationship as a symbol of hope that they want to destroy.
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.
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.
Perhaps the very magic that made Harry a god has a sort of negative side - effect. This could manifest as a shadowy entity within Harry that tries to take over his mind and use his powers for evil. So, in a way, this internal struggle could be the main antagonist in the story.