One interesting 'dark natsu nalu fanfiction' story could be about Natsu and Lucy facing a powerful dark magic that is corrupting their world. In this story, Natsu's dragon powers are affected in a strange way, and Lucy has to use her celestial magic in innovative ways to save them both.
You can try looking on fanfiction websites like FanFiction.net or Archive of Our Own (AO3). They have a wide variety of fanfiction, including 'dark natsu nalu' ones. Just use their search functions and type in 'dark natsu nalu' to find relevant stories.
One interesting Nalu dark fanfiction could be where Natsu is under a dark spell and Lucy has to find a way to break it. In this story, Natsu might be forced to fight against his friends, and Lucy's love for him is put to the ultimate test as she battles not only the external threats but also Natsu's inner darkness.
One of the main themes is family. The addition of a baby in the Natsu - Lucy relationship brings out a whole new aspect of family life. It shows how they support each other as parents.
Another great one is 'Baby Natsu: The Next Generation'. In this, the baby Natsu has some unique powers that are explored throughout the story. The author does a great job of creating a world where the baby Natsu is the center of attention while still keeping the essence of Natsu and Lucy's relationship intact.
Love is a very common theme. Writers often explore the romantic relationship between Natsu and Lucy, showing how their feelings develop over time. Adventure is also common. They go on quests together, fight bad guys, and discover new places.
Another type of 'dark nalu fanfiction' could be set in a world where Natsu loses his memories and becomes a part of a dark guild. Lucy then has to infiltrate the guild to bring him back to his senses. It's full of action, suspense and of course, the underlying relationship between Natsu and Lucy.
One common theme is the contrast between Natsu's fire magic and the cold elements in the story. This contrast often creates interesting situations and challenges for the characters.
First, understand the characters well. Know Natsu's impulsive nature and Lucy's determination. Then, create a dark scenario that challenges their relationship. For example, you could make a dark guild target Lucy to get to Natsu. Use vivid descriptions of the dark settings, like a gloomy forest or a haunted castle.
Another option is to check out some Fairy Tail fanfiction review blogs. These blogs sometimes feature and review 'dark nalu fanfiction' stories. They can give you an idea of which ones are worth reading based on the reviewer's opinion.