The fanfiction can create new plotlines. If it reveals something unexpected about Natsu's past, like a connection to a dark magic or a hidden enemy, it could lead to new adventures and conflicts within the Fairy Tail universe. It might also change the way other characters view Natsu, leading to different interactions.
It could change his relationships. For example, his friends in Fairy Tail might look at him differently. Some might be afraid, while others would try to support him more. Natsu himself might feel a sense of isolation as he tries to come to terms with what it means to be END. This internal conflict can drive the plot in many different and interesting ways in fanfiction.
It could reveal a lot. For example, we might learn about his early relationships, like with Igneel. Maybe we'd see how he developed his love for fighting and his loyalty to Fairy Tail. It could also show his struggles as a young dragon slayer, which would give us a deeper understanding of his current personality.
Well, this fanfiction could depict Natsu's power evolving during the fight. Maybe he discovers new ways to use his fire magic because of the intense battle with Jellal. Jellal's power might be shown as having hidden depths. His spells could have secondary effects that are only revealed as the fight progresses, which is different from how his powers are usually shown in the main series.
If Natsu is a girl in this fanfiction, her character might change in terms of her relationships. For instance, she may be more self - conscious about her appearance compared to when she was male. Also, her interactions with male characters like Gray could shift. Instead of their typical rival - like banter, there could be a more complex emotional undertone, like a friendly - rivalry with a hint of romantic tension perhaps.
In this fanfiction, the story might take a darker turn. Natsu is like the heart and soul of Fairy Tail in many ways. Without him, there could be a sense of gloom in the guild. However, this could also be a catalyst for change. Characters like Wendy could step up more. She might start to explore and master her dragon slayer magic in ways she hadn't before. And the relationships between the characters would shift, perhaps becoming more complex as they deal with the absence of Natsu.
Fanfiction about 'Fairy Tail' watching the past could involve the characters using some sort of magic or device to view their own past events or the history of their guild. It might show how they react to seeing their former selves, old relationships, and past mistakes.
It can create a lot of tension. If Natsu is mean to everyone, it will surely strain the relationships within Fairy Tail. This can lead to some characters leaving or trying to find a way to change Natsu.
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.
He might initially be shocked and confused. Natsu has always been so loyal to Fairy Tail, so this would be a huge blow to him.
It can add new plot elements. For instance, Envy's presence might introduce new conflicts that the Fairy Tail characters have to face. This could change the direction of some story arcs in the original Fairy Tail.