Another theme is self - discovery. Castiel in these omega fanfics usually goes through a journey of understanding his own omega nature. He might discover new powers or abilities related to being an omega, or he might learn more about his place in the world. This can be really interesting as it gives a new dimension to the character that we might not see in the original source material.
There's also 'Castiel's Omega Journey'. In this fanfic, it delves deep into Castiel's emotions as an omega. It shows his struggle to fit in while still maintaining his own identity. It has some really heart - touching moments that fans of the character will surely love.
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.
Arya Stark is sometimes reimagined as an omega in fanfiction. Her skills as a faceless assassin could take on a new dimension in an omega - centric story, using her stealth and ability to change identities to navigate the challenges of being an omega in a harsh world.
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.