Well, in many fanfictions, their relationship might start with Natsu being a bit in awe of Mavis as she is a very important figure in Fairy Tail's history. Natsu, being his brash self, might initially make mistakes around her but gradually learns from her wisdom. As time goes on, he could come to respect her even more deeply and they might form a bond where Mavis becomes like a guiding light for Natsu in his adventures and growth within the Fairy Tail guild.
In fanfiction, Natsu and Mavis could be like a mentor - mentee relationship. Mavis being the first master of Fairy Tail has a lot of knowledge and wisdom. Natsu could look up to her and she could guide him in his journey, much like how Makarov guided him in the main story.
One unique plot point could be Natsu having to learn magic in a completely different way under Mavis' tutelage. Instead of his usual hands - on, instinctive approach, he has to study theory and history. Another could be Mavis using Natsu to revive some long - lost traditions of the Fairy Tail guild. Maybe Natsu discovers a hidden power within himself that was dormant until Mavis guided him.
One possible plot could be that there is some sort of misunderstanding or dark magic at play that causes Natsu and Mavis to fight. Natsu, being hot - headed, jumps into the fray without fully understanding the situation. Mavis, on the other hand, is trying to figure out a way to stop the fight without harming Natsu. As the battle progresses, they both start to realize that they are being manipulated and then team up to face the real enemy.
In some fanfictions, Natsu's dragon slayer magic and Mavis's Fairy Law could be combined in a unique way. For example, Natsu could use his fire to enhance the power of Mavis's Fairy Law, making it more destructive against their enemies.
In a fight between Natsu and Mavis in a fanfiction, it's really hard to say who would win. Natsu is extremely powerful with his dragon slayer magic, being able to take on some really tough foes. However, Mavis is the first master of Fairy Tail and has a great deal of wisdom and powerful magic herself. It could really go either way depending on how the fanfiction writer wants to portray the battle.
One possible fanfiction could be that Natsu and Mavis get married in a simple yet magical ceremony. Their friends from Fairy Tail are all present, and the guild hall is decorated with beautiful flowers. Natsu is nervous but excited, and Mavis looks stunning in her white dress. After the wedding, they embark on a journey together to explore new lands and discover hidden magic, all while growing closer as a married couple.
In this fanfiction, Natsu and Mavis' married future could be one where they build a small cottage near the guild. They have a peaceful life, with Natsu going on occasional guild missions while Mavis stays back to tend to their home and garden. Their days are filled with simple joys like cooking together and sharing stories by the fireplace at night.
One possibility could be that Natsu is on one of his adventures and stumbles upon a young Mavis in a magical forest. Their meeting could be filled with wonder as Natsu shows Mavis some of his cool fire magic tricks, and Mavis, in turn, shares some of her early knowledge of magic. It could be the start of an interesting friendship that defies time and space in the Fairy Tail universe.
Some fanfictions depict their relationship evolving as they participate in guild - wide events. For instance, during a festival or a celebration in the guild, they might have a conversation that reveals new aspects of their personalities to each other. Over time, these small interactions build up, and they become closer friends, or in some cases, the relationship might take on a more romantic undertone as the fanfiction writer chooses to explore that aspect.
Natsu's character development in this fanfiction might involve him becoming more responsible. With his end form, he has greater power and with that comes the need to use it wisely. He could start to think more about the consequences of his actions. For example, he might not rush into battles as blindly as he used to. Also, his emotional growth could be a part of it. He could become more in tune with his feelings towards his friends, especially Lucy. His loyalty to the guild could also deepen as he realizes his new form can be a great asset to protect Fairy Tail.