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.
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.
One possible idea for a fanfiction could be a story where Kinana and Cobra meet again after a long time. Maybe Kinana has developed new powers and Cobra is on a path of redemption. Their encounter could be filled with tension as they remember their past interactions, both good and bad.
I'm not sure if there is a specific fanfic named 'Cobra the Dentist' in the Fairy Tail fandom. There are a vast number of fanfics out there, and it could be a very niche or newly created one. You might want to check popular fanfic websites like Fanfiction.net or Archive of Our Own and search for it specifically.
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.
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.
Another theme is power exploration. Cobra has unique powers, and fanfictions often explore how he can further develop and use those powers in different situations. It could be in battles against new enemies or in helping the Fairy Tail guild in unexpected ways.
One interesting Fairy Tail Cobra fanfiction could be one where he teams up with unexpected characters from the guild to take on a powerful new enemy. This would show a different side of his personality as he has to cooperate rather than be his usual solitary self.