Maybe the OC has the power of precognition. They foresee the Avengers' downfall in a particular battle against a cosmic threat. So, they intervene. Using their overpowered skills, they are able to communicate with the Avengers on a psychic level, guiding their every move during the fight. This allows the Avengers to anticipate the enemy's attacks and counter them effectively, ultimately leading to their victory.
The plot could involve the overpowered original character (OC) having some unique ability like super speed and time manipulation. They might first notice the Avengers in a dire situation, say being overwhelmed by an army of powerful villains. The OC could then use their powers to quickly turn the tide. For example, they could freeze time, isolate the villains one by one, and defeat them easily, saving the Avengers in a spectacular fashion.
Well, the OC could be a being from another dimension with god - like powers. The Avengers are on the verge of losing against an enemy that can absorb their powers. The OC shows up and simply nullifies the enemy's power - absorbing ability. Then, with a wave of their hand, they send the enemy back to where it came from. After that, they might offer to train the Avengers to better handle such threats in the future.
By having a unique power set. For example, if the OC can create portals to any location or dimension. This would be extremely useful in a rescue mission for the Avengers. They could use these portals to quickly move the Avengers out of harm's way or bring in reinforcements from other places.
The plot could involve Castiel being summoned by a magical accident in the Avengers' universe. He then realizes that there is a great evil approaching, something that not even the combined might of the Avengers can handle. So, he decides to use his knowledge of the supernatural and his powers to help them. He might have to teach the Avengers about the existence of angels and other celestial beings to make them understand the true nature of the threat. Along the way, there could be some really interesting interactions between Castiel and the different Avengers members. For example, his stoic and otherworldly nature could contrast with Tony Stark's sarcastic and scientific mind, leading to some great dialogue.
Perhaps the Avengers are under some sort of mind - control spell. Spiderman, being immune to the spell for some reason, has to figure out how to break it. He might swing around, avoiding attacks from his brainwashed friends, and then find a way to disrupt the source of the mind - control. For example, he could web up a device that was emitting the mind - controlling waves.
Well, perhaps Harry uses his magic skills like the Patronus charm to fend off some dark force that is attacking the Avengers. He could also use his knowledge of potions to heal any injured Avengers.
A possible plot twist could be that Spiderman is actually from an alternate dimension where he has different powers. And these powers are the key to saving the Avengers. For example, he might be able to time - travel in this dimension and go back to prevent the event that put the Avengers in danger in the first place.
Maybe the Avengers are under a curse that is slowly draining their powers. Percy, being a demigod, has some knowledge of curses. He hears about the Avengers' plight and decides to help. He embarks on a journey to find the source of the curse, facing many challenges along the way. Once he finds it, he uses his powers and the help of the Avengers (even with their weakened state) to break the curse and save them.
One possible plot twist could be that the real danger was not what it seemed at first. For example, while the Avengers thought they were being attacked by a supervillain, it was actually a test set up by a greater power. And Peter discovers this truth while trying to save them. Another twist could be that Peter has to sacrifice one of his powers temporarily to save the Avengers, which changes his approach to being a superhero.
Well, the plot could be that the OC (Original Character) is reborn into the Avengers' world with some special powers. Maybe they start off as an unknown, but gradually gain the attention of the Avengers through their unique abilities, like being able to control an element that no one else can. Then they get involved in the Avengers' battles, helping them in unexpected ways.
Well, in this fanfiction, it could be that Spiderman uses his agility and web - slinging skills to outmaneuver the enemy. Maybe he creates a huge web shield to protect the Avengers from a powerful blast. His spider - sense could also alert him to hidden dangers that the Avengers might not notice, allowing him to save them at the crucial moment.
Often, the fanfictions involve Tony relying on his network of contacts. He might call in favors from other scientists or even some minor characters from the Marvel universe. There's also the aspect of him sacrificing something, like his own safety or his reputation, in order to save the Avengers. And of course, there are epic battle scenes where Tony uses his Iron Man tech to fight off the enemy and protect his teammates.