One possible storyline could be that she is turned into a card by a powerful villain as a way to imprison her powers. Her friends then embark on a quest to find the magic to turn her back. Another could be that it was an accidental spell by a novice wizard that changed her, and she has to learn to communicate from within the card to get help.
Well, perhaps in the fanfic, she was turned into a card in a game world. The game has certain rules and she has to figure out how to use her card - form to her advantage. For example, she might be able to influence the game from inside the card and help her team win battles, all while trying to find a way to break the spell and return to her normal form.
In this fanfic, it could start with her being turned into a card during a magical experiment gone wrong. She finds that in card form, she can travel to different dimensions within the cards. She meets other 'card - beings' who have been trapped for ages. Together, they try to find a way to reverse the process for all of them. Along the way, she discovers new powers she never knew she had and uses them to fight against the forces that initially turned her into a card.
Maybe the main antagonist is a jealous rival. This rival is envious of her abilities and status, so she uses a forbidden magic to turn her into a card. The rival then tries to take her place in the story, but her friends start to suspect something is wrong and begin to investigate.
One possible storyline could be that as Spiderman turns into a girl, he - or rather she - has to deal with the sudden change in how society views her. She might face different challenges in her superhero duties, like people not taking her as seriously at first. Another could be that she discovers new powers or abilities that are unique to her female form.
Another storyline might involve Harry having to hide his condition from Voldemort and his Death Eaters, as being in a vulnerable pregnant state makes him an even bigger target. He could also form unexpected alliances with female characters who help him through this difficult time.
One possible storyline could be Naruto being possessed by a Devilman - like entity. His friends then have to find a way to free him while dealing with the new, more dangerous Naruto. Another could be Naruto willingly accepting Devilman - like powers to defeat a greater threat, but then struggling to control them.
Alucard could also be part of a story where he tries to find his place in the DC world. He starts in a small town in Metropolis and gradually makes his way into the larger superhero community. He might have to prove himself to heroes like Superman, who is initially skeptical of him. Along the way, he discovers a conspiracy within the LexCorp that involves using vampire - like creatures for evil purposes, and he has to stop it.
Perhaps in the fanfic, Sakura betrays her comrades due to a misunderstanding. She might be framed by an enemy as if she has leaked important information. This leads to her being ostracized by her friends, and she has to fight to prove her innocence while dealing with the emotional turmoil of the false accusation.
One possible storyline could be that Naruto is kidnapped by a rogue ninja group who wants to use his power for their own evil deeds. They might try to force him to reveal the secrets of his special jutsu. Another could be that he is kidnapped by a mysterious organization that believes he is the key to a long - lost prophecy. They hold him captive while they try to figure out how to use him to fulfill the prophecy. A third storyline could be that Naruto is kidnapped by a group of enemy ninjas from another village who see him as a threat to their plans for domination.
One possible storyline could be about a political intrigue where a foreign agent is kidnapped in America by a shadowy group with their own agenda. Another could involve a missing person case that is initially thought to be a simple disappearance but turns out to be a kidnapping related to some dark secrets in American society. Or perhaps it could be about a character from another country who is kidnapped in America for ransom by a criminal organization.
One possible storyline could be about Gwen facing the challenges of being a superhero while pregnant. She might have to deal with villains who try to take advantage of her condition. Another could be her preparing for the arrival of the baby, like setting up a nursery with her friends and family's help. And also, there could be a story where she discovers new powers related to her pregnancy that help her protect herself and the unborn child.
One possible storyline could be that Barry and Kara are in an alternate universe where they are the only two heroes left. In a moment of desperation and closeness, they share a kiss as a sign of hope and connection.