A common theme is survival. In the endverse, everything is in chaos and characters have to fight to stay alive. Another theme is the loss of hope. The world is so desolate that it's easy for the characters to feel hopeless.
One great supernatural endverse fanfiction is 'The End of All Things'. It really delves into the dark and gritty world of the endverse. The characterizations are spot - on, especially of Dean and his struggle in that desolate future.
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.
If we're talking about Castiel from 'Supernatural', his story is complex. He is an angel who rebels against Heaven's orders. He becomes involved with the Winchester brothers, Sam and Dean, and helps them fight various supernatural threats. He has his own journey of self - discovery, dealing with his loyalty to Heaven, his newfound friendships with humans, and the struggle between good and evil in the context of the show's universe.
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.