There's also a story where Castiel kidnaps a fan in a desperate attempt to save the world. It's a complex plot that involves time travel and alternate universes. You can find it on FanFiction.net. The story really delves into Castiel's character and his motives behind the kidnapping.
One common theme is the theme of rescue. Usually, the other characters, like Dean and Sam, will go to great lengths to save Castiel. For example, they might face numerous dangerous creatures or powerful enemies in their quest to rescue him.
A common plot is that Castiel kidnaps someone he believes is key to saving Dean or Sam. For example, in many fics, he might think a fan has some sort of special power or knowledge. Another plot is that he's being manipulated by other forces to kidnap someone, and then he has to figure out how to make it right.
A lesser - known but very interesting kidnapped Castiel fanfiction is 'Finding Castiel'. It has a unique take on the kidnapping plot, where Castiel is not only physically kidnapped but also has his memories manipulated. The way the author unfolds the mystery and the other characters' efforts to bring him back is quite engaging.
They might find an ally within the place where they are held. There could be another being there, maybe an angel who doesn't agree with Castiel's actions, or a human who has also been captured by Castiel in the past and knows the layout well. Together, they could come up with an escape plan.
One possible plot twist could be that Castiel kidnapped the character not out of malice but to protect them from a greater threat that the other characters are unaware of. For example, there might be a powerful and hidden enemy that has targeted this particular character, and Castiel believes that by taking them away, he can keep them safe.
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.
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.
Another excellent fanfiction is 'Heaven's Gatekeepers'. This one plays with the idea of their roles in the grand scheme of things in the Supernatural universe. It has a great plot that weaves in and out of canon events, and the way it portrays Dean and Castiel's connection is both unique and captivating.