In this fanfiction, a group of jealous sea creatures could be the antagonists. They don't like the fact that the ocean spoon girl is spending so much time with the lake fork guy. They try to sabotage their relationship by creating misunderstandings between them. For example, they might spread false rumors that the lake fork guy is only using the ocean spoon girl to get access to special ocean resources.
The antagonists could be a powerful storm spirit that is angry because the balance between the lake and the ocean is being disrupted by the friendship between the lake fork guy and the ocean spoon girl. The storm spirit creates huge and dangerous storms in both the lake and the ocean, endangering the lives of all the creatures. The two protagonists must find a way to appease the storm spirit and restore the balance.
One possible plot could be that the lake fork guy, who is used to the calm and fresh water environment of the lake, somehow gets transported to the ocean. There he meets the ocean spoon girl. She helps him adapt to the harsher, saltier ocean environment. They go on adventures together, exploring the ocean's depths and facing various sea creatures. Along the way, they develop a deep bond and friendship that might turn into something more romantic as they learn about each other's worlds.
One possible main antagonist could be Dr. Robotnik's sister, who is even more devious and has a different set of robotic creations. She could be jealous of Sonic's speed and popularity and wants to take over the world just like her brother, but with her own style.
There could be some traditionalist ninja in the village who oppose Naruto because she's a girl trying to break the norms. They might see her as a threat to the traditional male - dominated ninja culture and try to stop her from achieving her dreams and rising in the ranks of the ninja world.
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.
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.
A new student who is overly competitive and tries to undermine Ginger at every turn could be an antagonist. This student might be jealous of Ginger's popularity or her skills in a particular area like academics or a club activity.
Maybe it's a group of thieves in a fantasy - like city. The girl witnesses them stealing from an old lady and decides to intervene. She takes them on and manages to beat them up, but her glasses are knocked off during the fight. The thieves could be part of a larger criminal organization that might come after her later for interfering.
It could be some water - dwelling dark creatures. Maybe mermaids with a malicious streak or some sort of lake - bound demons that were awakened when Hermione got too close. They might see her as an intruder and want to harm her, which is why Harry has to save her.
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.
Doc Ock could also be a great antagonist here. He could be after some new technology or power source, and Spider Man and Spider Girl get in his way. He might use his mechanical arms to try and capture them or stop them from interfering with his evil schemes. Plus, his intelligence makes him a formidable foe for the two spiders.