One possible plotline could be how Harry and Carlisle meet in a world where the two fictional universes collide. Maybe they meet during a magical event that draws vampires and wizards together.
Another common theme is the concept of identity. Harry is known as 'The Boy Who Lived' and has to deal with his fame and destiny. Carlisle, on the other hand, has to balance his vampiric nature with his more human - like morals. Fanfictions may delve into how they come to terms with their identities, and how being in each other's presence might affect or change their self - perception. There could also be themes related to family, as both Harry and Carlisle have complex family situations in their original stories.
Regarding the Harry Potter and Carlisle Cullen mpreg fanfiction, it might seem quite far - fetched at first. But fanfiction is all about taking the known elements of different universes and twisting them in new ways. This could lead to some really emotional and thought - provoking stories. There could be a lot of internal struggle for Carlisle, as a vampire who is now facing a very human - like experience. And Harry could bring his own experiences of growing up in a difficult situation in the wizarding world to help Carlisle through this. It could also be a story that challenges the readers' perceptions of gender and reproduction in these fictional worlds.
One interesting fanfiction could be about Carlisle Cullen visiting Hogwarts. Maybe he goes there to share his knowledge of medicine with the Hogwarts staff. In this story, he could interact with characters like Dumbledore, and they could exchange ideas about magic and vampiric abilities.
One possible plot could be that a dark magic experiment gone wrong causes Carlisle to become pregnant. Harry, with his knowledge of various magics from the wizarding world, tries to find a way to reverse it or at least ensure a safe pregnancy. Along the way, they face opposition from both the vampire coven and some in the wizarding world who see this as an abomination.
They could interact in a way where Carlisle imparts his knowledge of self - control and morality to Harry. Given Carlisle's long existence as a vampire, he has a different view on life and death compared to Harry. This exchange of ideas could lead to some interesting character development for Harry. For instance, Carlisle might share stories of his past mistakes and how he overcame them, which could inspire Harry in his own battles against evil.
Yes, there are some out there. You can try searching on popular fanfiction websites like Archive of Our Own or Fanfiction.net. Just be aware that the quality can vary greatly. Some might focus more on the relationship development between Percy and Carlisle, while others could be part of a larger crossover plot with other characters from both universes.
One possible plot could be that there is a dark magic threat that endangers Carlisle Cullen. Harry Potter, with his knowledge of magic and experience in fighting dark forces, somehow travels to the Twilight universe and uses his spells to protect Carlisle from the threat.
Common themes often include the exploration of their personalities. Carlisle is a very old and wise vampire, while Harry is a young wizard with a lot of courage. Fanfictions might focus on how these traits play into their relationship. Also, the idea of secrecy and the challenges it brings in their relationship. Since they come from different worlds, they may need to keep their relationship a secret, which can create a lot of drama and tension in the story.
Fanfiction.net is also a great place to search. There are many authors who post their 'Harry Potter Carlisle Cullen' fanfictions there. However, the website's interface might be a bit old - fashioned compared to AO3, but it still has a wealth of stories. Just be sure to read the reviews and ratings to find the good ones.