Well, first of all, Natsu would probably be excited to see himself in all kinds of wild adventures in the fanfiction. He loves action and if the fanfic has him in some epic battles, he'd be all for it. Lucy might be a bit more critical. She'd look at how she's portrayed in terms of her intelligence and her relationships with the other guild members. Gray would likely be cool about it, but might get a bit annoyed if the fanfic made him seem too cold or too goofy. And Erza would be very strict in her assessment. If the fanfic doesn't adhere to the values of the guild she holds so dear, she might not be too happy about it.
The guild would likely be shocked at first. They consider Natsu as one of their own, so his attack would be completely unexpected. Some members might try to talk him down, like Lucy. She has a close bond with Natsu and would probably try to reason with him.
Natsu would be heartbroken. Fairy Tail is like his family. He'd probably go on a solo journey to clear his name and prove his loyalty. Erza would be stoic but deeply hurt. She'd start an investigation within the guild, using her influence and intelligence to uncover the truth. She might also isolate herself a bit while she figures things out.
The members of Fairy Tail would likely be shocked and angry. They would feel betrayed because Naruto had probably become a part of their family. Natsu, for instance, would be enraged and might immediately challenge Naruto to a fight. He's hot - tempered and wouldn't take kindly to someone he considered a friend turning against the guild.
Fairy Tail would be a mix of reactions. The younger members would be thrilled and look up to Naruto as a hero. The older and more experienced members would assess his skills and how he could fit into the guild. They might also be a bit protective of their way of life in the guild at first, but as they get to know Naruto better, they would accept him as one of their own.
The characters in Fairy Tail would have a wide range of reactions. Makarov, as the guild master, would be very cautious. He would need to ensure that Acnologia's presence doesn't put the guild in more danger. Happy would be constantly on guard around Acnologia, ready to fly away at the first sign of trouble. The Thunder Legion would likely be in a state of high alert. However, some of the more adventurous members like Cana might see it as an opportunity for great adventures and powerful new magic. It would be a complex situation with different characters having different stances based on their personalities and past experiences.
Some of the more cautious members, like Erza, might be a bit wary at first. She would be concerned about how Igneel's presence might change the power dynamics within the guild and what his intentions truly are. But over time, as she sees his positive influence, she would likely warm up to him.
Gray would be shocked at first. He and Natsu have a long - standing rivalry - but also a deep friendship. He might initially be angry that Natsu would just leave like that, but then he would realize how much he'll miss him. He might also feel a sense of loss for the guild as a whole, since Natsu is such an important part of it.
Lucy would be completely devastated. She and Natsu have a very close bond. She might go into a deep depression, constantly blaming herself for not being able to save him. Gray would be angry at first, feeling like he should have been there to help Natsu. Then he'd be sad and would probably vow to avenge Natsu. Erza would be strong on the outside but inside she'd be broken. She might try to keep Fairy Tail together in Natsu's memory. Happy would be inconsolable, constantly flying around looking for Natsu and refusing to believe he's really gone.