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.
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.
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.
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.
Perhaps as the Fourth Master, Natsu discovers some long - lost secrets about Fairy Tail. These secrets could be related to the origin of the guild or some powerful magic that was hidden away. He then has to figure out how to use this knowledge to the guild's advantage without causing chaos.
Obviously, Natsu is a very popular character as it's in the title. He is often the center of many fanfictions. Lucy is also quite popular as she has a close relationship with Natsu in the original series. Their dynamic often plays a big part in fanfictions.
In some fanfictions, he is a mentor - like figure. He teaches Mavis or other young wizards important lessons about magic, life, and the values of Fairy Tail. This could include things like the importance of friendship in magic, which is a central theme in the Fairy Tail series.
In fanfiction, the idea of Mavis and Zeref having a child often leads to very complex and interesting storylines. Some fanfictions might explore the child's powers, which could be a combination of Mavis's magic and Zeref's unique abilities. For example, the child could have an extraordinary control over life and death magic, or possess some form of enhanced intelligence due to their parents' natures.
In fanfiction, Mavis' brother can be portrayed in various ways. Some might depict him as a protective figure, always looking out for Mavis before her misfortunes. He could be a strong magic user, with unique abilities that are different from Mavis' own. For example, he might have a more combat - oriented magic compared to Mavis' strategic and leadership - related magic.
Natsu could bring a more fiery and passionate leadership style. He'd probably be more action - oriented than the previous masters. For example, if there's a conflict with another guild, he'd be the first one to charge into battle, leading by example.