In the fanfic, perhaps there's a prophecy that somehow involves their relationship. Reyna's crush on Percy could be a key factor in fulfilling the prophecy. They could be sent on a quest together, and during the quest, Reyna's feelings grow stronger. Along the way, they meet other characters from the series who either help or hinder Reyna's attempts to deal with her crush. It could also explore how Percy reacts if he eventually finds out about Reyna's feelings, whether he reciprocates or not, and how that would change the dynamics between them and the different camps in the Percy Jackson universe.
It might start with Reyna realizing she has a crush on Percy. Then, she could start to notice little things about him that she finds endearing. But there could be obstacles like her own pride as a strong leader in the Roman camp, and she doesn't want to seem weak by having a crush. So she tries to suppress it, but as time goes on, it becomes harder and harder to do so. This could lead to some really interesting character development for Reyna, and also some tense moments between her and Percy as he may or may not be oblivious to her feelings.
The plot could involve Reyna trying to hide her feelings from Percy while they are on some sort of adventure together. Maybe she has to work with him to defeat a new threat in the Percy Jackson world, all the while struggling with her internal emotions.
You can try looking on popular fanfic platforms like Archive of Our Own (AO3) or FanFiction.net. Just search for the relevant tags like 'Reyna', 'Percy', and 'crush' to narrow down the results.
One possible plot could be that after a great battle, Percy and Reyna find themselves drawn to each other more than ever. They start to work together on rebuilding their world, and during this process, their friendship blossoms into love and they decide to get married.
One possible plot twist could be that Percy is initially oblivious to Jason's feelings. Then, just when Jason is about to give up, Percy discovers his own hidden feelings for Jason. Another twist could be that there's some external force, like a god or a prophecy, that tries to keep them apart. Or perhaps Jason discovers that Percy has a pre - existing relationship that he didn't know about, which complicates his crush.
One possible plot could be that Jason is really shy about his crush at first. He tries to be around Percy more often, like joining the same quests or training sessions. He might secretly do nice things for Percy, like leaving his favorite food at his cabin without saying who it's from.
Maybe in the fanfiction, Percy and Reyna meet during a time of peace. They initially bond over their shared love for their respective camps. They decide to exchange knowledge, Percy teaching Reyna some of the more unorthodox combat techniques he learned from Poseidon, and Reyna teaching Percy about Roman military strategies. As they spend more time together, they also help each other deal with personal issues. Percy helps Reyna open up more about her feelings towards her sister, and Reyna helps Percy better understand his role as a hero in a changing world. Their friendship grows as they support and understand each other more deeply.
Well, it could start with Zoe being really shy around Percy. Maybe she blushes every time he talks to her. Then she might start leaving little hints about her feelings, like always being around where he is.
Maybe it could start with Blackfire secretly admiring Robin from afar. She might observe his heroic deeds and gradually develop feelings for him. Then she could start finding excuses to be around him more often.
The plot might involve Daphne's internal struggle. She has this crush on Harry but is afraid of being rejected. So she first tries to befriend Harry's friends to get closer to him. She could also be competing with other girls who are also interested in Harry. As the story progresses, she might gather the courage to confess her feelings directly to Harry.
One possible plot could be that Percy is being controlled by some dark force. Maybe a new evil god or an ancient curse. This force makes him kidnap Reyna because she has some important knowledge or power that the dark side wants. For example, she might know the location of a powerful weapon that could tip the balance in a coming war between the gods and the titans' remnants. Percy, against his will, takes Reyna to a secret location. Reyna, being a strong and resourceful character, tries to figure out a way to break Percy out of the control while also dealing with the situation at hand.
One possible plot could be that after years of adventures and growing closer as friends, Percy and Reyna start to realize they have deeper feelings for each other. They might face some initial resistance from their friends who are used to them just being comrades. But as they work together on a new mission, their bond strengthens and they finally decide to get married. The wedding could be a grand affair with all their demigod friends in attendance, celebrating their union.