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.
Another theme is friendship. His relationships with the other characters from Supernatural and Ghostfacers are often explored. How he bonds with Sam, Dean, or the Ghostfacers crew is a big part of many fanfictions. In some, he might form a really strong friendship with a new character introduced in the fanfic world.
In addition to the Ghostfacers as a group, individual members like Ed and Harry could be considered main characters. They often lead the group's investigations. When they meet Castiel, their personalities clash in interesting ways. Ed might be overly eager to learn about Castiel, while Harry could be more cautious.
The Ghostfacers would probably be excited. They are always eager to encounter new elements in the supernatural world, and an angel like Castiel is a huge deal. They might start asking him all sorts of questions right away. Castiel, on the other hand, might be a bit bemused. He's used to more serious situations and these guys with their cameras and loud voices could be a bit much for him at first.
There might also be a story where the Ghostfacers end up helping Castiel in an unexpected way. Perhaps they have some knowledge of a particular haunted location that is relevant to Castiel's mission. At first, Castiel is skeptical of their abilities, but they prove themselves useful in the end.
There's also 'Castiel's Redemption'. This fanfic focuses on Castiel's journey of self - discovery. It shows his interactions with the other characters from Supernatural and Ghostfacers in a very engaging manner. It's full of emotional moments and great character development.
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 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.
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.