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.
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.
Well, in a fanfiction scenario, if Bella were to be pregnant in a story involving Klaus and Marcel, it could create a whole lot of drama. Maybe Klaus would be jealous of Marcel's closeness to Bella. Or perhaps Marcel would see the baby as a threat to his relationship with either Bella or Klaus. It could also lead to a power struggle between Klaus and Marcel over the unborn child.
One possible plot could be that Teddy has grown up without a strong father figure and as he gets to know Harry better, he starts to see Harry as a dad. Maybe Teddy has some troubles at school or in his personal life and Harry steps in to offer guidance and support, which makes Teddy feel like he finally has a real father.
I'm not sure of a specific 'teddy calls harry dad fanfic' off the top of my head. You could try searching on fanfic platforms like Archive of Our Own. There are likely to be many fan - created stories there that might match what you're looking for.
Another theme is growth. Megumi's growth as a person and as a sorcerer is often tied to his relationship with Gojo. In these fanfics, Gojo's influence helps Megumi become stronger, not just in terms of his abilities but also in his emotional maturity. For instance, Gojo might teach Megumi how to handle difficult situations, which in turn helps Megumi grow. And of course, the theme of love is there. It might not be a romantic love, but the love between a father and son. Gojo's care for Megumi and Megumi's acceptance of Gojo as a father figure is a central part of these fanfics. It gives a warm and fuzzy feeling to the readers.
I'm not sure about the name exactly, but there are some fanfics where Megumi calls Gojo dad in a more action - packed setting. For example, in some where they are fighting curses together. Megumi's respect for Gojo as a father - like figure is shown through their teamwork and how Gojo protects Megumi during battles.
One possibility is that Klaus would try to claim the baby as his own, regardless of who the father really was. He's a possessive character, and having a child with Bella could be a way for him to have more power or control. Marcel, on the other hand, might be more concerned about Bella's well - being.
Snape might be initially shocked. His face could turn pale and his eyes widen in disbelief. But then, a small glimmer of something like pride or warmth might flash across his face for a split second before he quickly composes himself and snaps at Harry to focus on the potion.
The villains in Harry Marcel fanfic can also be really interesting characters. They are often complex, not just pure evil. For example, a villain who has a tragic backstory but still makes wrong choices. And then there are the wise old mentors in the fanfic who guide Harry Marcel on his journey, like an old wizard or a knowledgeable hermit.