Zeref, of course. He is a central figure in the Alvarez Empire. His power and his role as the emperor or leader make him a crucial character to include.
One idea could be exploring the backstories of the members in the Alvarez Empire. For example, showing how they joined the empire and what their motives were.
One interesting story could be about Naruto joining the Alvarez Empire and having to adapt to their different culture and ways of doing things. Maybe he has to prove himself in their combat trials.
One idea could be exploring the backstories of the Alvarez Empire soldiers. Maybe there's a young soldier who has doubts about the empire's motives and ends up befriending a Fairy Tail member. This could lead to a story of self - discovery and changing loyalties.
One possible plotline could be that Naruto is somehow corrupted or misled. Maybe he discovers some dark secrets within Konoha that he believes can only be resolved by its destruction. As for the Alvarez Empire, they might see an opportunity to expand their influence in the chaos Naruto creates and form an uneasy alliance with him at first.
Zeref is also a favorite. His connection to the overall story of Fairy Tail and his role in the events related to the Alvarez Empire offer a lot of room for creativity. Writers can explore his complex relationship with other characters like Natsu and how it impacts the empire - Fairy Tail dynamic.
Alliance building is also a theme. Alvarez might form alliances with different ninja villages or groups in the Naruto world. This could be for mutual benefit, such as sharing resources or fighting a common enemy. The fanfics that focus on this theme show how these alliances are formed, maintained, or broken, and how it affects the overall balance of power in the Naruto universe.
The Hokage, whoever it is at the time, would see Naruto's actions as a major threat. They would likely assemble the ninja forces to defend Konoha. But there would also be a part of them that would wonder what drove Naruto to this. Kakashi, for example, would be conflicted. He has a deep bond with Naruto, but he also has a duty to protect Konoha. So he would try to find a way to end this without harming Naruto too much.
Alvarez Kelly is a fictional tale. It doesn't draw from real events or people. The story was crafted by the filmmakers to engage and captivate the audience.