A common theme in such fanfictions is Natsu's unwavering courage when facing Acnologia. He might not be as strong as Acnologia at first, but his will to protect his friends and the Fairy Tail guild drives him forward. His dragon - slayer magic could potentially have some hidden aspects that are only revealed during this encounter.
In many fanfics, their relationship is often based on mutual respect. Natsu respects Erza's strength and leadership, while Erza respects Natsu's tenacity. For example, in some fanfics, when they are in a battle together, Natsu will follow Erza's strategic plans without hesitation.
Entropy in a Fairy Tail fanfiction related to Natsu and Mirajane could refer to the chaos or disorder within their relationship or the situations they face. It might be used to add an element of unpredictability to their interactions, like in a story where Natsu's impulsive nature and Mirajane's more complex personality create a chaotic dynamic that is full of unexpected twists and turns.
Another way motion sickness is used in Fairy Tail fanfiction regarding Natsu is in relation to his relationships. His friends might tease him about it, which adds a fun dynamic to their interactions. Or it could be a plot device where he has to overcome his motion sickness to save someone, showing his growth and determination.
She would fit in very well. The Fairy Tail guild is all about family and acceptance. Given her parents are Lucy and Natsu, she'd be welcomed with open arms. She could learn from the older members like Makarov and Mirajane, and bond with the younger ones like Romeo. Her magic skills would also contribute to the guild's strength during battles or when helping the town.
In this fanfiction, the story might take a darker turn. Natsu is like the heart and soul of Fairy Tail in many ways. Without him, there could be a sense of gloom in the guild. However, this could also be a catalyst for change. Characters like Wendy could step up more. She might start to explore and master her dragon slayer magic in ways she hadn't before. And the relationships between the characters would shift, perhaps becoming more complex as they deal with the absence of Natsu.
One possibility is that their child inherits Natsu's dragon - slayer magic and Lucy's celestial spirit magic. Maybe the child uses this unique combination to protect Fairy Tail from a new, powerful enemy.
Maybe the fanfiction could start with Natsu waking up not knowing it's his birthday. As he goes about his day in the guild, he starts to notice little things. People are acting a bit strange, whispering and giggling. Then suddenly, they all shout 'Happy Birthday' and the party begins. There could be a friendly sparring match as part of the celebration, with Natsu showing off his new moves he's been working on. And of course, there would be lots of fire - Natsu's signature - in the form of some cool firework - like spells created just for the occasion.
One possibility is that their daughter has a special ability to combine Natsu's fire magic and Lucy's summoning magic in unexpected ways. For example, she could summon a fire - based celestial spirit. She might grow up in a Fairy Tail that has changed a bit since her parents' time, with new members and new challenges. She could be mentored by some of the older guild members like Erza or Gray. And as she comes of age, she might discover secrets about her family's past that put her in a position to make important decisions for the future of the guild and the magical world at large.