One common theme could be the clash of their personalities. Beyond Birthday is rather unhinged and Light has his own sense of justice and superiority. So, the exploration of how they interact with their different mindsets can be a popular theme.
Another theme might be a power struggle. Light has the Death Note and Beyond Birthday has his own 'abilities'. Writers may explore who would be more dominant if they were to face off or work together. It's an interesting concept as both characters are quite complex in terms of their capabilities and motives.
You can try looking on fanfiction websites like Archive of Our Own (AO3). There are often a lot of fan-created works there. Just search for 'Beyond Birthday x Light' in the search bar, and it should show you relevant fanfictions if any exist.
One common theme is the exploration of different relationships. For example, fanfictions often re - examine the relationships between vampires and werewolves. Another theme is the expansion of the supernatural world, like introducing new types of magic or creatures.
A third common theme is self - discovery. Both Matsuda and Light are put in situations where they have to re - evaluate who they are and what they want, especially in relation to each other. This could be in terms of their feelings, their goals, or their moral compasses.
One common theme is the power dynamic. Given Light's intelligence and Misa's possession of a Death Note, there's often a play on who has more power in the relationship. Another theme is love and obsession. Misa is clearly obsessed with Light, and this is often explored in different ways in fanfiction.
One common theme is friendship. The 'light' character and the reader often start as friends and then develop a deeper bond. Another is adventure, where they go on some exciting journey together.
Another theme is the exploration of their relationship in a non - antagonistic way. Some fanfictions might show them forming an unlikely alliance or friendship, delving into how they could get along despite their differences. This often involves a lot of character development and emotional exploration. For example, they might start to understand each other's motives and pasts better. In some cases, it could even lead to a romantic relationship, which is also a theme in some fanfictions. Here, the authors might explore how their personalities would interact on a more intimate level, like how their shared intelligence could play a role in their romantic connection.
One common theme is loyalty. Mikami's unwavering loyalty to Light is often explored in different ways, like how far he would go to protect Light's plans. Another theme is power dynamics. Since Light has a certain hold over Mikami, fanfictions often play with this aspect.
Friendship is a very common theme. Tails' friends like Sonic, Knuckles, etc., usually play a big part in making his birthday special. They plan parties, give him gifts, and just show how much they care about him.
The interesting part could be how the writer interprets the relationship dynamics within the context of the 'beyond birthday' world. They might create unique scenarios and character interactions that are not present in the original source. However, it's important to note that 'lemon' usually implies some form of romantic or sexual content, which might not be to everyone's taste and also needs to be appropriate in the context of the fanfiction community.
The theme of friendship is likely prominent. Characters may rely on each other, support one another through difficulties. Also, the theme of redemption could be there. Some characters might be seeking to atone for past mistakes or wrongdoings in the story.