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.
The unique interactions. When Fairy Tail characters watch Natsu vs Zero, their reactions add depth. For example, if Lucy gasps or looks worried, it shows her care for Natsu.
One exciting moment could be when Natsu first unleashes a new and powerful attack against Zeref. Maybe it's a combination of his dragon slayer magic and some newfound strength he has gained. Another exciting part might be when Zeref counters with a really dangerous and dark spell that Natsu has to struggle to avoid. And of course, the moment when the Fairy Tail members react with shock or excitement to these powerful exchanges between the two would also be thrilling.
In this fanfiction scenario, one possible thing that could happen is that Natsu's determination to defeat Zeref might be put to the ultimate test. Natsu has always been a fighter with a strong will, and facing Zeref, he would have to dig deep. Zeref, on the other hand, being extremely powerful, might initially have the upper hand. But Natsu could tap into some hidden power, perhaps related to his dragon slayer magic in a new and unexpected way. Maybe he would get some kind of emotional boost from his friends in Fairy Tail who are watching, which gives him the edge he needs to turn the battle around.
One of the most exciting moments is when Natsu's intense fire magic clashes with Jellal's powerful Heavenly Body Magic. The visual display of their powers colliding is just amazing. Another great moment could be when they exchange some intense verbal sparring before the actual fight, adding more tension to the encounter.
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.
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.
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.
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.
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.