The World Government could still be the main antagonists. Since Luffy is getting stronger under Mihawk's training, they would see him as an even greater threat. They might send their top agents, like the Cipher Pol units, to try and stop Luffy's progress.
Another possibility is that some of the old foes of Luffy, like the remaining members of a defeated pirate crew that he once fought, come back for revenge. They see his training with Mihawk as a chance to strike while he is in a period of transition and not fully adapted to his new skills. They might team up with other villains in the One Piece world to create a more formidable force against Luffy.
Blackbeard could also be a main antagonist. He has his own ambitions for power and the One Piece. With Luffy getting stronger under Dragon's training, Blackbeard would view Luffy as a direct competitor. Their paths are bound to cross, and Blackbeard would not want Luffy to achieve his goals before he does.
One possible plot could be that Luffy, realizing he needs to become stronger in a different way, seeks out Mihawk. Mihawk, seeing potential in Luffy, decides to train him. Luffy would have to endure intense swordsmanship training, which is quite different from his usual combat style. This would also lead to some interesting character development as he struggles to adapt to Mihawk's strict training regime.
In luffy x mihawk fanfiction, the central characters are Luffy and Mihawk. Luffy, with his signature straw hat and his devil - fruit - given powers. He has a heart of gold and is always looking for adventure. Mihawk, with his black sword Yoru. He is a figure of great mystery and power in the One Piece world. Their different personalities and goals are what make their relationship in fanfiction so interesting to explore.
The Akatsuki could still be the main antagonists. Even with Naruto being trained by Minato and becoming godlike, the Akatsuki's goals of collecting the tailed beasts would put them in direct conflict with Naruto. Their powerful members, like Pain or Itachi, would pose a great challenge to Naruto, despite his new skills.
It could be some of the villagers who are jealous of Naruto's sudden growth under Shisui's training. They might try to undermine him or spread false rumors about him.
The Akatsuki could still be the main antagonists. Even with Naruto getting trained by Mangetsu, the Akatsuki's goals of collecting the tailed beasts would still pose a major threat to Naruto and the ninja world.
The Akatsuki could still be the main antagonists. Even though Sasori is training Naruto, the rest of the Akatsuki might view Naruto as a threat and try to eliminate him. Also, Orochimaru could be an antagonist. He might be interested in the new skills Naruto is learning from Sasori and try to steal them for his own experiments.
It could be a powerful pirate crew that has a vendetta against either Luffy or Sabo. Maybe they have some history with the Straw Hat Pirates or Sabo's group.
Often, the Marines can be main antagonists. They see pirates as a threat, and with Ace and Luffy being pirates, they would be constantly on the Marines' radar. Marines like Akainu are especially dangerous, as his extreme sense of justice could lead to direct confrontations with the brothers.
At first, Mihawk might see female Luffy as just another brash young pirate. But as he witnesses her selfless acts for her crew and her unwavering will, he begins to soften towards her. Meanwhile, female Luffy is constantly trying to break through Mihawk's cold exterior. Their relationship develops as they learn more about each other's pasts and what drives them. For example, when Mihawk reveals his own journey to become the world's greatest swordsman, female Luffy can relate it to her own dream of becoming the Pirate King. This shared understanding brings them closer and they form an unlikely bond that is a mix of mentorship and friendship.