It could be that Percy is being chased by some powerful enemy from his world and he stumbles into the Avengers' territory. His sudden appearance and the panic that follows draw the Avengers' attention. The 'atr' might be related to a special power or ability that Percy discovers in this new situation. This new power could be the key to defeating his pursuers, but he has to convince the Avengers to trust him first.
Obviously, Percy would be one of the main characters. He brings his own set of skills and experiences from his world. And of course, the Avengers. Maybe Tony Stark would play a major role as he is often the one to analyze new situations and technologies. Thor could also be important as he has some knowledge of magic and other realms which might help in understanding Percy and the 'atr' aspect.
Well, it could be about Percy being in a very stressful situation like facing a powerful enemy he's not sure he can defeat. This causes him to have a panic attack. Maybe he starts hyperventilating and his friends have to come to his rescue and calm him down.
Well, a great Percy Avengers panic attack fanfiction could be centered around Loki. Imagine Loki, who is always so in control, having a panic attack after a failed scheme. Percy, being a demigod used to the treachery of the gods, understands his state of mind. He uses his knowledge of Greek magic and psychology to calm Loki down. Or perhaps it's a story where the Avengers are facing a new threat that is somehow related to Percy's world. The stress of this unknown danger causes one of the Avengers, like Natasha, to have a panic attack, and Percy has to step in to assist.
One possible plot could be that Percy, who has some hidden motives or is being manipulated, leaks the Avengers' secret strategies to their enemies. Maybe he was promised power or something in return. This would lead to the Avengers facing unexpected and difficult battles as their foes seem to anticipate their every move.
One possible plot could be that the gods in the Percy Jackson universe sense a great threat from another dimension (the Marvel universe). Percy and his friends are sent to team up with the Avengers. Percy's water powers could be useful in fighting villains like Electro. Annabeth's strategic mind would assist in planning battles against foes like Loki. They all have to learn to work together, with Percy and the Avengers initially having some clashes due to their different ways of handling things.
One possible plot could be that in a world where the gods of Percy Jackson and the Avengers co - exist, Percy is somehow tricked or forced to banish the Avengers. Maybe there's a power struggle among the gods and they see the Avengers as a threat to their power, so they use Percy, who has a strong connection to the divine world, to get rid of them. Percy might be conflicted as he may admire the Avengers' heroism but has to follow the gods' orders.
Well, one plot could be that fem Percy's water - controlling abilities catch the Avengers' attention. Maybe she accidentally disrupts a city's water supply during a fight with a villain, and the Avengers try to figure out who she is and how to work with her to fix the situation.
To make it interesting, you can focus on the contrast between Percy's world and the Avengers'. For example, Percy's magic from the Greek gods is very different from the technology and superpowers in the Avengers' universe. When a panic attack occurs, this contrast can be highlighted. Maybe Percy's way of dealing with the panic attack is through some ancient Greek ritual, which the Avengers find strange but effective.
One possible plot could be that the Avengers mistake Percy for a threat. Maybe they see his powers with water and think he's some kind of new, unknown enemy. They capture him to figure out what he is. Percy, of course, would be confused and probably a bit angry at being wrongly accused.
One possible plot could be that the Greek gods, noticing the powerful threats the Avengers face, send Percy Jackson and his friends to help. Percy could use his water powers to assist in putting out fires during a battle like the one in New York. Meanwhile, Thor could be intrigued by Percy's connection to Poseidon and they could exchange stories about their godly fathers.