Well, it could be about Arianne being secretly betrothed to someone against her will. Maybe she has to navigate the politics of her family and the kingdom while trying to break free from this unwanted arrangement. For example, she might discover some dark secrets about her betrothed or the reasons behind the betrothal that put her in a dangerous position.
Obviously, Arianne herself is a central character. Her betrothed would be another important character. He could be a noble from another land, perhaps with a dark and mysterious past. Then there could be Arianne's family members, like her father who might be the one arranging the betrothal for political gain. Her friends or servants could also be characters, they might help her in her attempts to deal with the situation.
An OC can impact Arianne's character development by being a role model. For example, if the OC is a fearless warrior, Arianne might learn to be more assertive in battle or in political negotiations. This could make her a more formidable leader in Dorne.
Another theme is identity. Since she is an original character (oc), her identity within the Targaryen lore is often explored. How she fits in with the known Targaryen traits and history is a big part of many stories.
Daenerys would likely be the most powerful in many ways. She has dragons, which are a huge advantage in a world like Westeros. Her military experience and her claim to the Iron Throne give her a lot of political power as well. She has led armies and conquered cities, so she has a great deal of influence.
Love and loyalty could be another theme. In a polygamous relationship involving an OC Targaryen, the characters would need to navigate their feelings towards each other. Arianne might bring her own set of values regarding love and loyalty, especially considering her family background. Also, the idea of heritage and legacy could be a theme. The Targaryen bloodline is important, and how new relationships through polygamy might affect the family's future could be explored in these fanfictions.
One common theme could be power dynamics. Given that Daenerys and Arianne are both strong female characters in their own rights, exploring how power is shared or negotiated in a polygamous relationship with an OC could be a big part of the story.
In fanfiction, an OC (Original Character) Targaryen in a polygamous relationship with Daenerys and Arianne can be portrayed in various ways. Sometimes, the OC is depicted as a powerful figure who manages to navigate the complex politics of Westeros while maintaining relationships with these two strong - willed women. Daenerys, with her claim to the Iron Throne and her dragons, might initially be wary of sharing power or a partner, but over time could come to see the benefits of a united front. Arianne, being from Dorne and used to different cultural norms regarding relationships, could be more open to the idea from the start.
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.