A storyline could be that Percy has been manipulated by a long - lost acquaintance who has ties to the Avengers' enemies. This acquaintance might have convinced Percy that the Avengers are not as noble as they seem. Maybe they showed him some doctored information about the Avengers' past actions that made Percy believe they were the bad guys. As a result, he betrays them, but later realizes he has been deceived when he discovers the truth on his own.
One possible storyline could be that Percy is forced to betray the Avengers due to some sort of mind control. Maybe an enemy has a powerful device or magic that compels him to act against the team. Another could be that Percy is misjudged by the Avengers. They think he has betrayed them because of some false evidence planted by a villain who wants to create chaos within the group. For example, a villain might make it look like Percy has leaked important information about the Avengers' plans.
Another reason could be a difference in ideology. Percy might have had a different view on how to protect the world compared to the Avengers. He could have thought that their methods were too soft or too harsh, and decided that by betraying them, he could implement his own ideas better. He might have seen an opportunity to take matters into his own hands and make what he believed were the right decisions for the greater good, even if it meant going against the Avengers.
They were probably shocked at first. I mean, they trusted Percy, so it would be a huge blow. They might have felt angry and deceived.
One possible storyline could be that Issei betrays the group due to a misunderstanding. Maybe he is framed for something he didn't do, like stealing a powerful sacred gear. This leads to his friends turning against him at first. But as the story progresses, he works to clear his name and expose the real culprit.
Perhaps there is a powerful artifact that both Spiderman and the Avengers are after. They end up in a conflict over it. Spiderman, with his street - smart ways, tries to outwit the Avengers and get the artifact for himself, believing he can use it for good in his own way.
In some fanfictions, the storyline might involve a mix - up of magic. Percy's powers from the Greek gods could be pitted against the dark magic of Voldemort's heir. For example, Percy's ability to control water might be used to counter the heir's fire - based dark spells. There could also be a storyline where Percy is sent into the wizarding world by the gods to stop the rise of Voldemort's heir because they fear the chaos it could bring to the entire multiverse. And of course, there could be a love story where a character from Percy's world falls in love with someone related to Voldemort's heir, creating a complex web of relationships.
One possible storyline could be Percy forming a band with his demigod friends. They could go on a tour across different mythical realms, performing for gods and monsters alike.
One possible storyline could be Percy leading a Titan rebellion against the Olympians. Since he has a connection to the Olympian side as well, it would create a very interesting internal conflict for him.
Perhaps there's a power - stealing device in the fanfiction. It's stolen by a villain and used on the Avengers, but Gwen is immune. So the Avengers, now with reduced powers, end up fighting Gwen who is trying to get the device back. Or it could be that Gwen discovers a threat to her world that the Avengers don't believe in, and they try to stop her from taking action, leading to a conflict.
One possible storyline could be that Desmond has a hidden power that only he and a secret villain know about. The Avengers initially suspect him of being a threat, but as the story unfolds, they realize he's been trying to protect them all along. Another could be that Desmond is from a parallel universe and his presence in the Avengers' world starts causing glitches in the space - time continuum.
One possible plot could be Percy trying to find a way back home. He might have to complete certain quests or tasks set by the Avengers or some other powerful entity in the fanfiction world. Another plot could be Percy getting involved in a conflict that he wasn't originally a part of because of his exile, like a battle between different superhero factions.