Mirajane is a popular choice too. She has a mysterious side to her, and in the fanfictions, she could use her unique abilities to help Natsu or be involved in the events that the Fairy Tail members are watching. Also, Laxus might be included. His powerful magic and complex character can add depth to the fanfiction stories, especially when his relationship with Natsu and the rest of the guild is explored.
Gray is another character. His ice - magic can complement Natsu's fire - magic. They might team up to fight enemies or overcome obstacles in the way of saving Fairy Tail. Sometimes they bicker while doing so, which adds some humor to the story.
One common plotline could be Natsu going on a solo adventure that the other Fairy Tail members watch through some magical means. Maybe he discovers a new type of magic that is both dangerous and alluring. Another could be Natsu facing a version of himself from an alternate timeline, and the guild watches as he battles his other self to save the world.
Often, it could be because of a false accusation. Maybe someone framed Natsu for a crime he didn't commit, like stealing a powerful magical artifact from the guild. So, to maintain peace and order, the guild leaders had no choice but to banish him.
Obviously, Natsu would be one of the main characters since the fanfiction is centered around him fighting. His friends from Fairy Tail like Lucy, Gray, and Erza are also very likely to be main characters. They would be the ones watching Natsu fight and having reactions to his actions in the battle.
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.
They have great chemistry. Natsu is the impulsive and powerful fire wizard, while Erza is the strong and disciplined armor wizard. Their contrasting personalities make for an interesting dynamic. They also have a long history in the Fairy Tail guild, which gives fanfiction writers a lot of material to work with.
Some common themes are redemption and forgiveness. Given Zeref's actions in the original Fairy Tail series, fanfictions might delve into whether Natsu could be the one to bring about Zeref's redemption. Family also plays a role. With their shared history, fan stories could explore the idea of them being part of an unexpected family or having a brotherly bond in a different way than what's shown in the official story.
I haven't read this particular fanfiction yet. But generally, in a scenario where Fairy Tail characters watch Natsu fight Zeref, it could be filled with intense emotions. Natsu's determination and his unique powers would be on full display. And the reactions of the other Fairy Tail members would add depth to the story, whether it's shock, pride, or worry.
Erza is frequently paired with Natsu in these fanfictions. Given her strong and leadership - like qualities, she can take charge in agent - related situations, and Natsu can be the more impulsive force. Their interactions can range from Erza scolding Natsu for his reckless actions to them having deep conversations about their responsibilities as agents. Wendy is also a character that can be paired with Natsu. Her healing magic can be crucial during dangerous agent missions, and she looks up to Natsu, which can create a sweet and protective relationship in the fanfiction.
In some fanfictions, Natsu might be developed as more of a strategist than in the original series. He could learn from Laxus' more calculating approach to battles. Laxus, on the other hand, might be shown to be more open - hearted, influenced by Natsu's warmth and loyalty to the guild.