Well, a great storyline might involve Hisui getting lost in a dangerous magical realm and Natsu being the one to go and rescue her. During their time in the realm, they rely on each other's strengths. Hisui uses her knowledge and diplomatic skills, while Natsu uses his brute strength and dragon slayer magic. This shared experience brings them closer. Or perhaps there's a prophecy about Hisui and Natsu that they initially don't believe in, but as events unfold, they start to see the truth in it.
One great example could be those that explore their relationship in an alternate universe. Maybe a world where magic is used in different ways. Another is the ones that focus on how they first meet in the fanfiction world. It could be a chance encounter during a magical event. And there are also fanfictions that deal with them facing a common enemy together and how that experience brings them closer.
They often interact with Natsu being his boisterous self and Hisui being intrigued by his energy. For example, in some fanfictions, Natsu might show Hisui a new type of magic, and Hisui would be both amazed and a bit cautious.
For Hisui and Natsu's character development, it's important to give them growth arcs that are in line with their existing traits. Hisui could start off as a bit reserved and formal, but as she spends time with Natsu, she becomes more open - minded and adventurous. Natsu, on the other hand, could learn more about responsibility and respect for authority through his relationship with Hisui. You can use different plot events like battles or diplomatic meetings to showcase these changes in their characters. Also, let their relationship progress gradually, from initial distrust or indifference to mutual respect and affection.
To develop their relationship realistically, start with small interactions. For example, they could meet at a guild event and have a short conversation. Then, gradually build on these interactions. Maybe they have a common goal, like helping a village that has been affected by a dark wizard. As they work towards this goal, they start to understand each other's strengths and weaknesses.
One great fanfiction is 'Natsu and Lisanna's Adventure'. It beautifully portrays their relationship development. It starts with a simple quest they embark on together, and as they face challenges, their bond deepens. The author does a wonderful job of showing Natsu's protective side towards Lisanna and Lisanna's admiration for Natsu.
Hisui could be put in situations where she has to make tough moral choices. For example, when faced with a decision that could benefit the kingdom but harm an innocent group of people. This would force her to think deeply about her values and develop her character in terms of ethics.
Another idea is that the OC is from a different land and has a completely different view of magic compared to the people in the Fairy Tail world. Hisui, as someone interested in different cultures and magics, is intrigued by the OC. They start a journey to understand each other's worlds better. During this journey, they encounter various magical creatures and have to overcome cultural misunderstandings and the opposition of some who don't like the idea of mixing different magics.
The unique character of Hisui. She has her own charm and backstory in the Fairy Tail universe. Writers can explore her character in different and often exciting ways in lemon fanfiction, adding a new dimension to the existing story.
One great 'Fairy Tail Natsu x Harem' fanfiction is 'A New Adventure'. It has an interesting plot where Natsu finds himself in various unexpected situations with the harem members. The character development is well - done, especially how Natsu's relationships with each of the harem characters evolve over time.