A cool plot could be that they are on a quest to find the source of all elements. Along the way, they face various challenges like crossing a lava river (which is hard for Water Girl) and a frozen lake (tough for Fire Boy). They learn more about each other's powers and limitations, and their friendship grows stronger as they progress on this difficult journey.
Maybe they encounter a third character who has the power of earth. This earth - powered character tries to separate them because their combined powers are too strong. Fire Boy and Water Girl then have to find a way to convince the earth - powered one that they mean no harm and that together they can do good things, like helping the environment in their magical world.
One idea could be that Fire Boy and Water Girl find a magical forest where the rules of their elements are reversed. Fire Boy has to be careful not to extinguish himself in the overly wet areas, and Water Girl has to avoid drying up in the super - hot spots. They have to work together to find a way out of this strange forest.
A possible plot is that the body swap happens on a special day. The boy - turned - girl has to go on a date that the girl had planned. He tries to act as the girl but makes some funny mistakes. The girl - turned - boy has to participate in a sports event the boy was supposed to be in. They both start to see the world from a different perspective and learn to appreciate what the other gender goes through.
Another antagonist could be a jealous fire spirit. This fire spirit is envious of Fire Boy's connection with Water Girl. It tries to turn Fire Boy against Water Girl by making him believe that she will eventually put out his fire forever. So it creates misunderstandings between them to try and break their friendship.
One interesting plot idea could be a story where a dragon from a minor tribe rises to become a great leader and unites all the tribes against a new, powerful threat that none of them could face alone. This would involve the dragon facing many challenges and prejudices along the way due to their tribe's status.
One idea could be exploring a different path for the main characters. For example, if they met under different circumstances, like at a different event or in a different kingdom. This could change the initial dynamic between them.
One interesting plot idea could be a story centered around an underdog character in the Fire Emblem universe. This character starts off as a low - ranked soldier but through determination and unique strategic thinking, rises to become a key figure in the war. Another idea is a fanfic that explores the backstory of a minor villain, showing what led them to their evil ways.
One idea could be a story about a new recruit who has a hidden past as a firefighter in a different city and has to prove themselves to the Chicago Fire team. They might face challenges like adjusting to the different procedures in Chicago while also dealing with personal issues from their past.
One idea could be exploring the backstory of the main character's magical powers in more depth. Maybe she discovers a hidden origin of her powers that changes her perception of herself.
One idea could be a magical spell gone wrong that turns a girl into a boy, and she has to navigate through a world that treats her differently now. For example, in a school setting, she might find that she's suddenly expected to participate in boys' sports or activities. Another plot could involve a scientific experiment that causes the transformation, and she then has to find a way to reverse it while dealing with her new identity. Maybe she forms new friendships with boys who accept her as one of them, but she also misses her old female friends.
In a fantasy world, a boy is cursed by an evil witch to turn into a girl. To break the curse, he has to complete a series of quests. Along the way, he meets different characters who teach him about the different experiences and challenges that girls face, and he gains a new understanding and respect for the opposite gender.
The girl could be a journalist or a writer who wants to do an undercover story on what goes on in a boy camp. So she sneaks in. While there, she experiences the camaraderie among the boys, the tough training, and the unique traditions. She also has to deal with her growing feelings for one of the boys she befriends, which complicates her mission.