One common theme is the struggle between Natsu's vampiric nature and his love for Lucy. His vampiric instincts might drive him to do things that could potentially harm Lucy, and he has to constantly fight against that. Another theme could be the mystery surrounding his transformation into a vampire. Fans often write about how it happened and what it means for his place in the Fairy Tail guild.
In nalu vampire fanfiction, a frequent theme is the power dynamic. As a vampire, Natsu may be more powerful than before, which can change the way he and Lucy interact. Also, the theme of secrecy is quite common. Natsu might have to keep his vampiric identity a secret from the rest of the guild, and Lucy has to help him with that. This secrecy can create a lot of tension and drama in the story. Additionally, there could be themes related to redemption, especially if Natsu's vampirism is seen as a 'curse' that he needs to overcome in order to be with Lucy fully.
Well, many nalu vampire fanfictions explore the idea of forbidden love. Vampires are often seen as outcasts or dangerous beings in the fictional world. So, Natsu being a vampire and in love with Lucy is a form of forbidden love. This can lead to a lot of angst and emotional turmoil in the story. Another common theme is the exploration of the vampire mythology within the context of the Fairy Tail universe. How does Natsu's vampirism affect his magic? How does he fit into the world of wizards and mages now? These are all things that are often explored in such fanfictions.
The anticipation of the new life is another theme. Writers often describe how they both look forward to the arrival of their baby. They might dream about the baby's future or argue about baby names.
One common theme is domestic bliss. Writers often explore how Natsu and Lucy create a warm and loving home together. For example, they might show Natsu bringing home little trinkets for Lucy or Lucy cooking a special meal for Natsu.
One common theme is the clash of different power systems. Since Nalu comes from Fairy Tail with magic and Naruto has ninja abilities, seeing how they interact and compare is often explored.
Friendship is also a big theme. Even though the focus is on their potential romantic relationship, the foundation of their friendship is often explored. How they support each other as friends, share their joys and sorrows, and that friendship gradually evolving into something more is a recurring idea in nalu one - shots.
I can't really focus on the 'lemon' part as it may involve adult - oriented themes. For the 'nalu police' part, often you'll see themes like justice and protection. Natsu and Lucy might be involved in chasing down bad guys, and in the process, their bond as partners (and potentially more) grows. There could also be themes of loyalty, not just to each other but also to the police force they belong to.
A prevalent theme is the power - up or growth of the characters. In these fanfictions, Natsu and Lucy often discover new powers or ways to enhance their existing ones. Another theme is the exploration of their pasts. Maybe Natsu learns more about his dragon parent, or Lucy uncovers secrets about her family's magic heritage. These elements add depth to the characters and the overall story.
Love and friendship are common themes. Often, the relationships between the characters are explored in depth, whether it's a romantic relationship or a strong bond of friendship.
One common theme is the exploration of their relationship in different time periods. For example, seeing how their feelings for each other might have developed differently if they had met earlier or later. Another theme is the idea of changing the past to affect the future. In many nalu time travel fics, they might try to prevent a bad event from happening in the past, which could have a huge impact on their present world.
Another theme might be the unexpected situations that occur due to their drunken state. For example, they could end up in a place they've never been before or get involved in some comical misunderstandings. Also, the 'lemon' aspect might imply a bit of a more mature exploration of their relationship, like a new level of physical closeness or attraction that is explored in a tasteful way in the story.
One common theme could be young love. Since it's set in highschool, Nalu's relationship would be in its budding stage. Another theme might be friendship groups and how they influence Nalu's relationship.