The Marines could still be antagonists. They might view Luffy's transformation as a sign of weakness or an opportunity to capture her more easily. They could underestimate her because she is now a girl, but of course, they would be wrong.
One potential antagonist could be a jealous sorcerer. This sorcerer sees the special bond between Pearl and the mystery girl and wants to break it. He might try to use dark magic to turn the mystery girl against Pearl or steal her powers if she has any.
There could be a group of Pokémon poachers who mistake Ash, in his new female form, for an easy target. They might try to capture his Pokémon while he's dealing with the changes in his body and abilities. Or perhaps there's a jealous trainer in Erika's gym who doesn't like the attention Ash is getting, even in his new form, and tries to sabotage his attempts to turn back.
One potential antagonist could be a rogue ninja who also has aerokinesis but uses it for evil. He might be jealous of Naruto's natural talent and try to undermine him at every turn. Another could be a powerful sorcerer from a far - off land who has come to the Naruto universe specifically to find someone with aerokinesis to either steal their power or use them as a tool. There could also be a group of anti - ninja who see Naruto's new power as a threat to their plans of overthrowing the ninja system and they try to eliminate him.
Jealous characters could be antagonists. Maybe there are other girls who are envious of the beloved and pampered girl and try to bring her down. Another possibility is that her over - protective family members, in a sense, could act as antagonists if their pampering restricts her freedom too much.
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.
The Marines could be antagonists. They might see the baby as a threat or something that needs to be taken away from Luffy and Nami's crew. Maybe they think the baby has some connection to a dangerous pirate or revolutionary. The Marines would try to capture the baby and arrest Luffy and Nami for having it.
Perhaps there could be a plot where Luffy, as a girl, has to use different strategies in battles. Since the opponents might underestimate a female opponent, Luffy could use this to her advantage and surprise them with her strength. This would also show a different side of Luffy's combat intelligence.
Sakura could be a potential love interest. Since Naruto has always had a connection with her, as a girl, that connection could turn into a yuri - like romantic relationship. They already know each other well, and Naruto's new form could make Sakura see her in a different light.
Perhaps Orochimaru could still be an antagonist. In this fanfiction, he might be jealous of Naruto's new god - like gaming powers and try to steal them for his own experiments.