Who would be the most interesting students for Castiel to interact with in a 'Castiel teaches at Hogwarts' fanfic?

One possible plot could be Castiel trying to understand the unique magical system of Hogwarts. He might initially struggle as his angelic powers work differently. For example, he could be confused by the use of wands when he's used to his own celestial abilities. But as time goes on, he starts to blend his knowledge with the wizarding world's, creating new spells or teaching methods that combine angelic and magical concepts.

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.

He might be more open and affectionate with Sam and Dean. Usually reserved, the alcohol could make him more likely to give hugs or say things like 'You two are truly my best friends on Earth' in a slurred but sincere way.

The clash of different universes is really interesting. For instance, when Castiel is put into a world like Middle - earth from Lord of the Rings, his angelic nature stands out so much against the more earthly magic and races there.

Dean Winchester is often a common character. Since he has a strong bond with Castiel, he might be involved in the time - travel events either directly or through Castiel's actions in the past that affect him in the present.

Well, it depends on the type of fanfiction. If it's a romantic fanfiction about him, he might be really embarrassed. But if it's an adventure - type fanfiction that expands on the battles and quests in his world, he could gain some new perspectives on his own experiences. He may also start to understand how the fans view his character and the world he inhabits in a whole new way.

One of the main things is the exploration of his character. When Castiel is an archangel, it's not just about his powers. It's about how his identity changes. Does he still feel the same loyalty? How does he view his purpose now? Fanfics can dig deep into these aspects. They can also show how his new form affects his appearance and how other characters react to this change. This all combines to make the 'Castiel is an archangel' fanfics really interesting.

Rowena might also be a character in this fanfiction. She has a vast knowledge of magic. So, she could be the one who either accidentally turned Castiel into a dog in the first place or is the key to turning him back. Her sassy attitude would add a lot of flavor to the story as she deals with a dog - Castiel and the Winchesters.

A really interesting plot idea could be Castiel trying to save a character from 'Vampire Diaries' from damnation. Since he's an angel, he might see it as his duty. Another one is Castiel being mistaken for a new type of vampire because of his pale appearance and some of his otherworldly abilities.

Maybe the main villain is an ancient spirit that was once locked away in a dimension between the two worlds. Danny's accidental crossing of dimensions and Castiel's presence somehow break the seal. This spirit wants to destroy both the world of the living (that Danny protects) and heaven and hell (that Castiel is associated with) in order to create its own chaotic realm.