Jiraiya might also be a good choice. He's well - traveled, has a vast amount of knowledge about different ninja arts and cultures, and is also a great teacher. His flamboyant and adventurous nature would bring a unique flavor to the guild.
Kakashi could be a great guild master. He's already a highly skilled and respected ninja in the Naruto world. His knowledge of various jutsu and his experience in leading teams would make him a natural fit for the role of guild master.
Tsunade could be a strong guild master. She's tough, has great leadership skills as seen when she led the Hidden Leaf Village, and her medical ninjutsu knowledge would be useful for the guild's well - being. Also, her no - nonsense attitude would keep the members in line.
Maybe in the 'natsu guild master fanfic', Natsu becomes guild master during a time of crisis. A dark magic is spreading and threatening all the guilds. Natsu has to make difficult decisions, like forging alliances with former enemies. He uses his fiery determination to inspire his guild and they fight back the dark forces together. He also discovers new aspects of his magic along the way to better protect the guild.
Sasori's apprentice who has gone rogue and wants to create the most powerful human puppets ever. He believes that by using living people as puppets, he can achieve ultimate power. He would be a formidable antagonist as he has Sasori's knowledge and a thirst for more power.
Another great choice could be Laxus. He has that tough - guy exterior but also a lot of room for character growth. In a fanfiction as guild master, he could bring a different leadership style, more aggressive perhaps. He might change some of the guild's traditions while still maintaining the core values of Fairy Tail. He also has a great relationship with his team, which could be further developed in the fanfiction.
One possible interesting plot could be Naruto, as the guild master, having to deal with a powerful dark guild that is threatening the safety of Fairy Tail. He could use his ninja skills and leadership to gather the members and come up with a unique battle strategy.
Sakura would likely be one of the first members. She has been Naruto's friend for a long time and is a strong ninja in her own right. Also, Rock Lee could be an early member. His determination and unique fighting style would be great assets to the guild.
A plot could be about a new guild master having to deal with a powerful dark guild threat. The new guild master, say someone like a new character introduced in the fanfiction, has to unite the Fairy Tail members, who are initially skeptical of their leadership. They have to come up with new strategies to combat the dark guild's unique magic and overcome internal differences within Fairy Tail.
The dark guilds would be great antagonists. They might see Natsu's new leadership as a threat and try to undermine his guild. They could send their best mages to disrupt the guild's operations and steal important magical artifacts.
It could be the leader of the dark guild. This person would likely have a very different ideology from Naruto's. Maybe the leader believes in using extreme and unethical means to gain power, like sacrificing innocent lives for dark magic. Naruto would constantly clash with this leader as he tries to uphold his own values.
Sasuke could potentially be an infiltration master in fanfiction. He already has the Sharingan, which gives him an edge in observing and predicting movements. He could use his skills and his dark past to become someone who can infiltrate any place. Maybe he uses his knowledge of different ninja clans and their weaknesses to his advantage while infiltrating.
It could be a new original character created specifically for the fanfiction. Someone with a unique jutsu or skill related to trap - making.