A lot of these fanfictions explore the theme of identity. Leviathan Castiel is not the same as the Castiel Dean knew, so there's a journey of both of them trying to figure out who this new entity is in relation to their past connection. Also, the theme of trust is big. Dean has to decide whether he can trust this new form of Castiel, and that creates a lot of tension and drama in the stories.
One great Leviathan - themed 'Supernatural' fanfiction is 'The Leviathan Chronicles' on FanFiction.net. It delves deep into the nature of the Leviathans and their interactions with the Winchester brothers in a very engaging way.
One great aspect of Castiel and Dean in fanfiction is their dynamic. Their relationship, which is complex in the show, can be explored even further in fanfictions. Writers often delve into their unspoken bond, creating scenarios where they support each other in ways not shown in the original series. For example, some fanfictions might have them going on a road trip together just for the sake of exploring new places and having deep conversations about their pasts and futures.
Fanfiction about Castiel in Supernatural can be really interesting. Some 'hopeless' fanfictions might focus on his often tragic or difficult situations in the show. They could explore his loyalty to Dean and Sam even when things seem truly hopeless for him. Maybe it's about his struggle with his angelic nature and the chaos of the human world he's thrown into.
A really creative plot is in a fanfic where the Leviathans can possess animals as well as humans. It makes for some really strange and unexpected encounters for Sam and Dean.
Characterization is key. Since Leviathans are complex creatures in the Supernatural world, a good fanfiction will develop their characters well. For example, showing their intelligence and cunning. Another element is plot. It should be engaging and perhaps explore new aspects of the Leviathan lore that the show might not have fully covered.
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.
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.
I remember reading a fanfic called 'Castiel's Wounds'. In this story, Castiel is severely injured during a battle with demons. The fic focuses on how Sam and Dean try to help him recover, and the relationship dynamics between the three are really well - written. It shows Castiel's vulnerability in a very believable way. There's also 'A World of Hurt' which explores Castiel's hurt in a post - apocalyptic setting, adding an extra layer of tension and drama.
Well, 'The Road Less Traveled' is a popular 'Supernatural Dean and Castiel fanfiction'. It's set in a post - apocalyptic world where they have to rely on each other more than ever. It has great character development and some really intense scenes. There's also 'Soulmates in the Shadows' which plays with the idea of them being soulmates in a unique way.