One common theme could be forbidden love. Given the setting in which Oberyn Martell exists, a gay relationship might be frowned upon in that society, adding an element of secrecy and danger to the story.
I'm not sure about specific Oberyn Martell gay fanfics. But you can try searching on fanfiction websites like Archive of Our Own. They have a large collection of fan - made stories and you might be able to find some related to Oberyn Martell's gay relationships there.
There might be some Oberyn Cersei fanfiction out there, but the 'lemon' aspect makes it a bit tricky. Fanfiction websites usually have rules against explicit content. You could try looking for more general Oberyn Cersei fanfiction that focuses on their power dynamics, their possible interactions given their characters in the show or books, rather than seeking the 'lemon' type which is likely to be hard to find in a legitimate and clean - cut way.
Fanfiction.net is also a place to check. While it has a wide variety of content, you can usually find Oberyn Cersei fanfiction that is more about their relationship in the context of the 'Game of Thrones' plot. Look for tags like 'character study' or 'relationship exploration' rather than anything that might suggest adult content. Another option is to join 'Game of Thrones' fan groups on social media platforms and ask for recommendations for clean Oberyn Cersei fanfiction.
In a fanfic, if Oberyn wins, he may also decide to take Sansa Stark under his protection. He could see her as a key to future political stability. With Sansa in Dorne, he could use her claim to the North to form a coalition against the Lannisters, uniting the North and Dorne against their common enemy.
They can be combined by creating a story where Arianne Martell is a transfer student to Hogwarts. She comes from a different magical culture and has different magical techniques that she shares with the students at Hogwarts. Harry Potter could befriend her and they could learn from each other.
There might be a fanfiction where Arianne Martell is secretly a descendant of a powerful wizarding family in the 'Harry Potter' universe. Harry Potter discovers this connection and they embark on a journey to uncover the secrets of her family's past, which could be tied to some ancient magic or prophecy.
I'm not sure who the 'best' is as it's subjective, but some popular authors in the fanfiction community might be those who have a deep understanding of Arianne's character. They would be able to write her as a strong, independent woman who is true to her Dornish roots.
It gives fans a chance to expand on the world of Dorne that Arianne Martell is from. Writers can create their own versions of events, add more depth to the culture, and involve their OCs in political intrigue or romantic adventures. For example, an OC could be a foreign diplomat who has to deal with Arianne in matters of state, leading to all sorts of exciting scenarios.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). It has a large collection of fanfics, and you can search specifically for Arianne Martell OC ones. Just type in the relevant tags in the search bar.