One common theme is power. Since Castiel is an alpha, his power is often explored, like how he uses it to protect or lead. Another theme is loyalty, as in how his pack or those around him are loyal to him, and vice versa.
There's a fanfic called 'Alpha Castiel: The New Order'. In this, Castiel is an alpha in a post - apocalyptic world. It shows his leadership skills and his journey to protect his pack. It's full of action and interesting character dynamics.
Archive of Our Own (AO3) is another great place. It has a very active Supernatural fandom community. Just type in 'alpha dean x omega castiel' in the search bar and you'll be presented with a variety of stories. The site also allows you to sort by different criteria like popularity, word count, etc.
There's 'The Bond of Alpha and Omega'. This fanfiction focuses on how Dean's alpha instincts kick in when it comes to protecting Castiel, the omega. It's beautifully written, with detailed descriptions of their interactions and the growing bond between them. It also has some interesting side characters that add depth to the story.
Castiel's relationship with his powers might also change. He may have to limit his use of angelic powers to protect the unborn entity. This can cause internal conflict as he is used to using his powers freely. At the same time, it can also show his growth as a character, as he is willing to sacrifice some of his power for the sake of something new and precious.
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.
Harry Potter himself would be very interesting. Castiel could be intrigued by Harry's connection to Voldemort and his role as the 'Chosen One'. He might offer Harry some unique insights from his angelic knowledge about dealing with dark forces, which could be different from what Dumbledore has taught him.
In dean - castiel relationship in such fanfiction, there's often a deep emotional connection. They might show a side of extreme vulnerability towards each other. Their relationship can be intense, with a lot of passion and a sense of 'destined to be together' kind of feeling.
Yes, there are likely some Naruto and Castiel fanfictions out there. Fanfiction writers often like to combine different fandoms in creative ways. You can try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net to find such works.
A bit. Given that he is part of the 'official' narrative, fanfiction can be really out - there. It might seem odd to him that fans have taken his character and put it in all sorts of situations that may not be in line with what he knows. However, he might also find it interesting to see the different perspectives on his character.