Well, they might become even closer to the other guild members. After surviving such a harrowing experience, the other members could look at Natsu and Erza with newfound respect. Natsu might be seen as even more of a hero, and Erza as an even stronger protector. This could lead to stronger bonds within the guild, with the other members relying on them more in tough situations.
On the other hand, some guild members might become jealous of Natsu and Erza's new - found status. They survived when others might not have, and this could create some tension. However, Natsu and Erza being who they are, they would probably try to ease the tension by showing their comrades that they are still the same people and that they all contribute to the guild in different ways. This would be a complex change in relationships that would add depth to the fanfiction.
Their relationship with some of the younger guild members could change significantly. For example, they could become mentors to the new generation. Natsu might teach the younger ones some of his more unorthodox fighting techniques, while Erza could instill discipline and a sense of honor. This would change the dynamic within the guild, making it more of a family - like unit where knowledge is passed down from the more experienced members.
Erza would be more composed. She would understand Natsu's reasons to a certain extent. However, she would also see the importance of the guild unity. So she would try to balance between respecting Natsu's decision and finding a way to make him come back if possible. She might send out some members to keep an eye on Natsu from a distance.
Some of the younger guild members like Wendy would be a bit shy about it. They might blush and look away when they see Natsu and Cana together after the kiss. Makarov, the guild master, would likely be more accepting. He would see it as a natural part of the members growing up and forming relationships within the guild, as long as it doesn't disrupt the overall harmony of Fairy Tail.
The guild members' reactions would vary greatly. Happy, being Natsu's closest companion, would likely be excited about the fanfiction. He might fly around the guild hall, telling everyone about it and adding his own little comments about how accurate or inaccurate it is. He would definitely be protective of Natsu's image in the fanfiction, especially if it's not a positive portrayal.
There's also a possibility that it causes a rift in the guild. Other members might take sides. Some might side with Erza, believing she had a good reason, while others might side with Natsu, feeling that Erza was too harsh. This could create a lot of tension within the Fairy Tail guild and further complicate the relationship between Erza and Natsu.
A possible plot twist is that they survive but are transported back in time on Tenrou Island. They have to relive certain events, but this time with the knowledge they already have. This could lead to them changing the past in unexpected ways, like preventing some of the tragedies that originally occurred on the island, which would then have a huge impact on the future of the Fairy Tail guild.
In many fanfics, their relationships start as comrades in Fairy Tail. For example, in some stories, Natsu's protectiveness towards Lucy and Erza's respect for both of them are the initial building blocks. As the story progresses, they might go through a dangerous mission together where they rely on each other more deeply. This could lead to a stronger bond, like a family - like relationship.
In fanfiction, the relationship between Natsu and Erza's child and the other Fairy Tail members would be quite diverse. The child would be part of the Fairy Tail family. Wendy would be like an older sister, helping the child with their magic and always being kind. Cana might teach the child some of her card tricks and gambling skills in a more innocent way. Elfman would be the one to encourage the child to be manly, and Evergreen would be that strict but caring mentor figure. They'd all contribute to the child's growth and development within the guild.
Some of the more mischievous guild members like Gray might tease them a lot. He'd probably make jokes about it every chance he gets, much to Natsu and Lucy's annoyance. Happy, on the other hand, might be really confused at first but then start to see it as a sign of their closeness and be really happy for them.
Some members might be in shock. For example, Happy, who is Natsu's closest friend, would be extremely sad and might even consider leaving the guild too. He has always been by Natsu's side, and the sudden expulsion would leave him feeling lost.
In the fanfiction, their relationships could develop slowly. Erza might start by being more protective of Natsu during battles. She has always been a strong fighter, and now she has an extra incentive to keep Natsu safe. Mirajane, on the other hand, could use her charm and her skills in the guild to make Natsu feel welcome and special. As time goes on, their feelings might become more obvious to other guild members, who could either encourage or tease them about it.