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 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.
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 many Fairy Tail fanfictions, the battle between Natsu and Acnologia is often depicted as extremely epic. Natsu's fiery determination and his unique dragon - slayer magic make for an interesting contrast against Acnologia's overwhelming power. Some fanfictions might have Natsu discover new forms or powers in order to stand a chance against the fearsome Acnologia.
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.
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 the Fairy Tail members watching will have different reactions. Some might be worried for Natsu, especially those who are close to him like Lucy or Happy. Others, like Erza, might be more focused on analyzing Natsu's moves and Zero's counter - attacks. And there could be some comic relief from the likes of Gray, making snarky comments while still being engaged in the fight between Natsu and Zero.
One exciting moment could be when Natsu first unleashes his most powerful Fire Dragon Slayer attack against Laxus. The sheer power of the flames and the look of determination on Natsu's face would be thrilling. Another moment might be when Laxus counters with his Lightning Dragon Slayer magic, creating a spectacular display of light and energy as the two elements collide.
In my opinion, Natsu would have a good chance of winning. Natsu is known for his incredible determination and his ability to keep getting back up no matter how tough the fight. His Dragon Slayer magic gives him great power and his Fire Dragon's Roar can deal significant damage. Also, his experience in various battles has toughened him up. However, Laxus is no pushover either. He has his Lightning Dragon Slayer magic which is extremely powerful. But Natsu's never - say - die attitude might just tip the scales in his favor.