Well, another option is to check out some dedicated Supernatural and Avengers fan communities. For example, on Reddit, there are subreddits for both fandoms. You can post a request there asking for any 'Castiel Saves the Avengers' fanfiction, and the community might be able to help you out. They might know of some hidden gems or specific authors who write this kind of crossover.
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.
Some Tumblr blogs are also great sources for fanfiction. There are a lot of fans on Tumblr who are into both Supernatural and the Avengers. Search for relevant tags like 'Castiel Avengers fanfic' on Tumblr. However, Tumblr's search can be a bit finicky sometimes, so you might need to dig a little deeper by going through different blogs and following related tags.
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.
I think Steve Rogers would be a likely ally. Steve is all about justice and doing what's right, and Castiel has a similar sense of duty in his own way. They could bond over their strong moral compasses.
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.
I haven't actually read this fanfiction, so I can't summarize it specifically. But generally, it might involve Stiles, a character probably from another universe like Teen Wolf, using his unique skills or knowledge to rescue the Avengers who are in some sort of danger. It could be a really exciting and creative crossover story.
Sorry, I haven't read this specific fanfiction, so I can't summarize it directly.
It could be that a powerful sorcerer in the Avengers' world goes rogue and starts using dark magic. The Avengers don't know how to handle magic, so Harry is called in. He has to face this rogue sorcerer, using his own brand of light magic. Along the way, he might have some clashes with the Avengers' way of doing things, but in the end, they all work together and Harry manages to save the day by defeating the rogue sorcerer and restoring peace in the Avengers' world.
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 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.
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.
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.