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.
You can try searching on popular fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. Just type in 'Castiel Saves the Avengers' in their search bars, and you should get relevant results.
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.
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.
In a 'Castiel meets the Avengers' fanfiction, there could be a lot of interesting scenarios. Maybe Castiel tries to understand the Avengers' concept of heroism which is very different from his own in the Supernatural universe. He might be intrigued by their technology and try to figure out how it works in relation to the magic he is used to.
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, 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.
One possible plot could be that Castiel discovers a baby that has some sort of supernatural origin. Maybe the baby is a key to stopping an impending apocalypse. Castiel then has to protect the baby while also dealing with his angelic duties and the reactions of his friends like Dean and Sam.