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.
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.
In this fanfiction scenario, Matt Murdock could bring a whole new dynamic to the Avengers. His skills as Daredevil, like his enhanced senses and combat abilities, would be quite an asset. Maybe he'd initially clash with some of the more brash members due to his more reserved nature. For example, he might have a bit of a tense encounter with Tony Stark at first, but over time they could learn to respect each other's strengths.
Well, in the fanfiction, Castiel might be initially confused by the Ghostfacers' methods. He could be intrigued by their enthusiasm despite their somewhat amateurish approach to the supernatural. Maybe he'd end up helping them out in a case without really meaning to at first, like when they misinterpret a sign and Castiel corrects them, leading to a successful hunt.
One great 'Misha meets Castiel fanfiction' is 'The Unexpected Encounter'. It has a really interesting plot where Misha stumbles upon Castiel in a very unique situation. The story does a wonderful job of capturing their personalities.
In a 'Danny Phantom meets Castiel' fanfiction, perhaps Danny is drawn to Castiel's world because of some strange energy fluctuations. Castiel senses Danny's arrival and is intrigued. They start to communicate, with Danny explaining his world of the living and the dead, and Castiel sharing about heaven, hell and the angels. Eventually, they find out that there is a dark force trying to merge their two realms and they must stop it.
Well, it could be really chaotic. Draco's slytherin ways might clash with the Avengers' hero complex. He might try to manipulate the situation for his own gain, but the Avengers would likely see through his tricks.
The Avengers as a whole would likely be outraged at the idea of someone being abused. Captain America would lead the charge in trying to bring the abusers to justice. The team would come together to not only protect the reader physically but also help them heal emotionally. They would surround the reader with love and support, showing them that there are good people in the world who will fight for them.