A very prevalent theme is the idea of 'opposites attract'. Natsu is hot - headed and impulsive while Lucy is more level - headed and organized. Fanfictions play with this dynamic a lot. Also, the theme of sacrifice is common. They are often willing to sacrifice for each other, which shows the depth of their love.
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 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.
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.
Common themes often include the power of friendship. Since Natsu and Lucy are part of Fairy Tail, their friendship is a strong foundation. In the 'accidentally in love' aspect, the theme of unexpected events leading to love is prominent. For example, an accidental magic spell that makes them see each other in a new light. And of course, the theme of perseverance in love. They may face challenges from within the guild or from outside threats, but they keep coming back to each other.
One common theme is adventure. Since they are from a world full of magic and quests, their love often develops during adventures. For example, they might face a powerful enemy together and in the process, realize their deep feelings for each other.
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.