He could return with a whole new set of allies. During the war, he might have met other powerful mages who decided to follow him back to Fairy Tail. These new allies could bring different types of magic and skills to the guild. Also, Natsu might come back with a different perspective on life. The horrors of war could make him more determined to protect peace in the world and in his guild, leading him to take on a more leadership - like role in Fairy Tail.
Well, he could be magically healed by one of his friends. Maybe Lucy uses her celestial magic to call a spirit with healing powers.
The first thing that could happen is that his friends would immediately start to tend to his wounds. Erza, being the strong and reliable one, might take charge of the situation. She could use her medical knowledge, if she has any in the fanfiction world, or she could call for help from a more experienced healer. This would show the bond between Natsu and his friends.
Well, perhaps Ash comes back with new skills and a sense of purpose. He might be determined to protect Serena even more than before. In the fanfiction, he could use his war - learned strategies to deal with any threats that come their way. And their relationship could evolve into a deeper, more mature love as they face new challenges together.
Well, one possibility is that Ash has changed a great deal due to the war. Maybe he has new skills or a more hardened personality. He could struggle to fit back into his old life with Serana. For example, he might be haunted by the memories of the war and find it difficult to be as carefree as he once was with her.
One possibility is that Igneel would immediately go into a protective mode. He might use his dragon powers to heal Natsu if he has such abilities. Since they have a strong bond, Igneel would be enraged at whatever caused Natsu to get hurt and would seek revenge. He could also stay by Natsu's side until he recovers, providing warmth and comfort as dragons are often associated with being a source of strength and protection.
After the kiss, it could be that they both realize their true feelings deeper. Natsu might start to see Juvia in a whole new light, not just as a friend. Juvia, on the other hand, would be even more in love. Their story could then develop into a series of sweet moments. For example, Natsu could start bringing her little gifts, like a flower he found on a mission. And they could start training together more often, which would strengthen their bond not only as a couple but also as stronger wizards.
Well, one possibility is that Yukino is completely taken aback. She might not be expecting it at all. She could initially say no because she's not sure about her feelings or she might be worried about how it will change their lives. But then, as she thinks more about it, she realizes that she does love Natsu and they start planning a future together.
His friends would most likely try to heal him immediately. Wendy, with her Sky Dragon Slayer magic, could be the first to attempt to close his wounds and stop the bleeding. She has great healing abilities, so she would be crucial in this situation.
He might also have an emotional reaction. Spiderman is a very empathetic character. If the fanfiction shows some really tragic events during the Infinity War, he could be deeply affected. This could lead to him being more determined to prevent such tragedies from happening in his reality.
The fanfiction could start with Natsu's grand return. His friends are overjoyed but soon realize he's different. He might be haunted by the memories of the war. Maybe he lost some dear friends during the war and is now more determined to protect the ones he still has. His return could also trigger some rivalries. There could be those who are jealous of the newfound respect he has earned from the war, or those who think he has become too powerful and might be a threat. This would create a lot of drama and new storylines in the fanfiction.