The plot could be that the OC is a new duelist in the city. Akiza notices the OC's unique dueling style which is different from what she has seen before. They start as rivals but gradually develop a friendship as they learn from each other's dueling strategies.
Love is also a prevalent theme. The fanfictions often explore the romantic aspect of their relationship. They might go through the typical stages of a budding romance like the first blush of attraction, misunderstandings, and finally coming together. This can be seen in fanfictions where they have to overcome obstacles in their relationship like differences in their social status within the Yugioh 5ds world or misunderstandings about each other's intentions.
Friendship is also a big theme. The OC might start as a stranger to Luna but then they develop a deep friendship through shared experiences, like helping each other out in difficult situations or just having common interests in the world of duels.
One great piece of Leo x Luna fanfiction could be 'The Bond Beyond Dueling'. It beautifully portrays their relationship as it develops over time. The author does an excellent job of showing Leo's protective side towards Luna and how Luna's gentle nature influences Leo.
One popular place to find Yugioh 5ds Luna x OC fanfiction is Archive of Our Own (AO3). There are many stories there that explore different relationships between Luna and original characters. Just search for 'Yugioh 5ds Luna x OC' on the site.
Another theme is the concept of the outsider. Since vampires are not the norm in the 5ds universe, they are often seen as outsiders. The fanfictions explore how they try to fit in or change the existing order. They may face discrimination from other characters at first but then prove their worth through their dueling skills.
One popular theme is the exploration of the characters' backstories in more depth. For example, delving into Jack Atlas's past before he became famous. Another theme could be alternate universes where the events of the 5ds story take a different turn, like if the Signers didn't exist. And of course, there are often themes centered around the duels themselves, with new and unique decks being created by fanfic writers for the characters to use.
Yes, there are quite a few. You can check out some fanfiction websites like FanFiction.net. There you might find various stories exploring the relationship between Jack and Yusei in different ways, from deep friendships to more complex emotional bonds.
One common theme is the clash of dueling styles. Since GX and 5DS have different rules and strategies in dueling, fanfictions often explore how characters from both series adapt to each other's ways. Another theme is friendship. Characters from the two shows come together and form new bonds. For example, they might start as rivals but end up as friends through their dueling experiences.