One common theme is the exploration of their different natures. Jacob being a werewolf and Carlisle being a vampire. Writers often play with how they overcome their natural enmity and form a relationship.
Fanfiction.net is also a place to look. However, the tagging system might not be as precise as AO3. But if you dig a bit, you may come across some interesting Jacob x Carlisle fanfictions there. Just be sure to check the ratings and reviews to ensure the quality.
Another theme is friendship. Many fanfics show them gradually becoming friends, like in 'The Friendship Forged', where they start off as enemies but end up having a deep bond.
One aspect of Carlisle Cullen x Bella fanfiction could be the exploration of their unique relationship within the Twilight universe. Carlisle is an old and wise vampire, while Bella is a young human. Fanfictions might delve into how Carlisle's self - control and moral code interact with his growing feelings for Bella. It could also show how Bella views Carlisle in a different light compared to Edward, perhaps seeing him more as a father figure or a source of guidance at first, and then developing more complex emotions.
One great Carlisle and Jacob fanfic is 'The Unlikely Bond'. It explores their relationship in a unique way, with Carlisle's wisdom and Jacob's impulsiveness creating interesting interactions.
One common theme is the exploration of their different worlds. Harry comes from the wizarding world and Carlisle from the vampire world in Twilight. So, stories often deal with how they bridge that gap.
The theme of sacrifice also frequently appears. Either Carlisle or Bella may have to sacrifice something they hold dear for the sake of their relationship. It could be Carlisle's standing in the vampire community or Bella's normal human life. These sacrifices add depth to their relationship in the fanfiction stories.
There's also 'The Bond with Carlisle'. The OC in this one, Sophia, is a vampire hunter who falls in love with Carlisle. It's an interesting take as it goes against the normal dynamic. The story delves into how they overcome their differences and build a relationship. The fanfiction has detailed descriptions of their emotions and the struggles they face, making it a great read for Carlisle Cullen x OC fans.
In Carlisle x Harry fanfiction, a prevalent theme is the discovery of each other's strengths. Harry is known for his magical abilities and courage, while Carlisle has his vampiric powers and moral compass. Their mutual respect for these qualities can be a central theme. Another common theme is the idea of protecting each other. Given their respective dangerous worlds, they might find themselves in situations where they need to safeguard one another. Additionally, the theme of cultural exchange between the vampire and wizarding cultures can be quite interesting, as they learn about each other's ways of life and values.
Character - wise, the fanfictions might develop Bella and Carlisle in different ways. Bella could be portrayed as more assertive in her feelings for Carlisle, and Carlisle might struggle more with his vampiric nature and his love for Bella. The 'lemon' aspect can add a layer of sensuality to the story, making it a more intense exploration of their relationship compared to regular non - 'lemon' fanfictions about them.