If the fanfiction is set in a different timeline or universe, it could be that Akeno herself is corrupted by a dark power source. In this case, Akeno would be the main antagonist in a sense, but the real source of the problem could be the dark power that has changed her and made her want to hurt Naruto.
It could be an evil spirit that has possessed Akeno. This spirit might be using her to get to Naruto because it sees Naruto as a threat to its plans, whatever they may be. So the main antagonist would actually be this hidden spirit.
In this fanfiction, Akeno is likely a character who has some sort of relationship with Naruto, either a friend or a love interest, who then does something to break his heart.
It could be that Akeno is forced by an enemy to target Naruto. She might use her powers to harm him, and Naruto has to find a way to both avoid her attacks and free her from the enemy's control. This would create a complex situation where Naruto has to balance self - defense and saving Akeno.
One possible antagonist could be a powerful wizard or sorcerer who is jealous of Naruto's growing influence and the harem he has. This antagonist might try to break up the group and steal their powers.
Well, obviously Naruto and Akeno are the main characters. Naruto, the energetic and determined ninja from the Hidden Leaf Village. And Akeno, who has her own set of powers and a distinct personality from her own fictional universe. They are the focus of the fanfiction, and the story usually revolves around their interactions, whether it be a friendship, a partnership, or a romantic relationship.
Perhaps in a fanfiction, Naruto and Akeno are both chosen to participate in a grand tournament that involves fighters from different universes. They train together, sharing their skills and knowledge. During the tournament, they face powerful opponents and have to rely on each other's support. Their bond grows stronger as they progress through the tournament, and they become not just partners but also very good friends.
Naruto could initially be shocked and hurt. Given his trusting nature, Akeno's betrayal would come as a big blow. He might go into a state of disbelief, trying to figure out why she would do such a thing.
One possible fanfiction could be about Naruto and Akeno meeting in a cross - over universe. Maybe Naruto is on a mission in Akeno's world and they end up teaming up. They could learn from each other's powers and fighting styles. For example, Naruto could show Akeno his Rasengan while Akeno could demonstrate her magic to Naruto.
A group of traditionalists in Akeno's world could be antagonists. They might not approve of the idea of a harem or of an outsider like Naruto having so much influence. So they could try to break up the harem and drive Naruto out of their world by using political means, like spreading false rumors about Naruto and his friends.
Perhaps they are guardians of a peaceful village that is suddenly under threat from an ancient evil. Naruto, with his experience in protecting his own village in the Naruto series, and Akeno, with her combat skills from High School DxD, must work together. They start by training the villagers to defend themselves, and then face the evil directly. Along the way, they might have to overcome differences in their fighting styles and personalities to succeed.
Their relationship would become more centered around the baby. They would have to communicate more about parenting decisions, like how to train the baby if it has special abilities. Their love would grow deeper as they see a part of each other in their child.