Yes, there are likely to be such fanfics. Fanfic writers often explore unique relationships in their works, and the idea of Marcel calling Klaus 'dad' can add an interesting dynamic to a story. You can try searching on fanfic platforms like Archive of Our Own or Fanfiction.net.
One possible fanfiction could be about Caroline, a human, being drawn into Klaus' world in a small town. Maybe she discovers his true nature as a vampire gradually. At first, she's scared but then becomes intrigued by his complex personality.
Caroline as a human in fanfiction with Klaus can create a really interesting dynamic. It allows for exploration of how a human might interact with a powerful vampire like Klaus. For example, she could be a source of light in his dark world, constantly challenging his views.
The plot could involve Marcel having a crisis where he realizes that his biological family has let him down in some way, and Klaus, who has been there for him in many situations, becomes the person he looks to as a father. Maybe Marcel gets into trouble with other vampire factions, and Klaus is the one who steps in to save him, leading to this newfound father - son relationship.
One common theme is redemption. Klaus, who has a dark past, often seeks redemption through his relationship with Caroline in these fanfictions. For example, in many stories, he tries to change his ways for her.
Well, Klaus is typically depicted as a powerful and mysterious figure. He has his own motives and desires which often collide with the human world. Caroline, on the other hand, is the one who brings out his more human side. She might be a student, or have some other normal human occupation that contrasts with Klaus' otherworldly existence. There may also be some supporting characters from the original story's universe who play roles in either helping or hindering their relationship.
One common theme is love against all odds. Since they often come from different backgrounds in these fanfictions, like in some where Klaus is from a wealthy but cold family and Hayley from a middle - class, warm family. Their love has to overcome these differences.
At first, he might be shocked and a bit scared. But then, Klaus would probably become extremely possessive and overprotective. He'd use his power to surround Caroline with security. He might also be a bit more tender towards her than usual, as he realizes the importance of the new life they are about to bring into the world.