One interesting story could be where Carlisle's love for Bella is a slow - burning one. He tries to resist it at first because of the complications it might bring with Edward and the rest of the family. But as time goes on, he realizes he can't help his feelings.
In some fanfictions, Carlisle's love for Bella might be portrayed as a more forbidden love. Maybe it starts with him being her doctor and having this instant connection. This could lead to a lot of internal conflict within Carlisle as he has to balance his feelings with his role in the vampire family and the vampire world.
One common theme is the exploration of forbidden love. Since Carlisle is an older vampire and Bella is a human (initially), their relationship goes against the norms of both the vampire and human worlds.
One popular 'bella falls in love with carlisle fanfiction' is 'A Different Kind of Love'. It explores a unique take on their relationship, with Bella being drawn to Carlisle's wisdom and kindness in a more profound way than in the original story.
He becomes more vulnerable. Usually, Carlisle is seen as this strong, composed leader. But when he falls in love with Bella, his emotions start to get the better of him at times. He might second - guess his decisions more often.
In the fanfiction, Carlisle's sense of duty to his family and the vampire code might be a hurdle. He has to balance his feelings for Bella with his responsibilities to the Cullen family. Also, Bella's own human life and her relationships with other humans, like her father, could be a complication. How can she maintain those relationships while being in love with Carlisle? There's also the issue of secrecy. Their relationship has to be kept hidden from the outside world to protect both of them.
One interesting fanfiction could be about Bella having special powers that Carlisle discovers and they go on a journey to understand them. It might involve them traveling to different vampire covens to seek knowledge.
One interesting story could be where Carlisle has to protect Bella from a new threat in Forks while dealing with his complex feelings for her. In this fanfiction, their relationship might develop in a more angsty way compared to the original story.
One popular theme is the forbidden love aspect. Since Carlisle is much older and in a different position in the vampire world compared to Bella, this creates a sense of the relationship being taboo, which makes for an interesting story.
One interesting plot could be that Dark Carlisle has some hidden powers that Bella discovers. This discovery could lead to a series of adventures where they have to face off against some new and powerful enemies in the vampire world.
One possible plotline could be Carlisle teaching Bella more about the history and ethics of the vampire world. For example, he might take her on trips to ancient vampire sites and share stories passed down through the ages.
Some fanfictions might explore a plot where Carlisle and Bella are forced to be together in a situation where they have to rely on each other for survival. Maybe they are trapped in a dangerous place filled with other menacing vampires or some other supernatural threat. This would lead to them bonding on a deeper level and their relationship evolving in a way that is different from the typical courtship.