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.
Natsu's injury could also lead to a discovery of new powers or abilities. As he is on the verge of death or in a very weakened state, his body might instinctively tap into some latent magic. This could be a game - changer in the fanfiction, not only for Natsu's character development but also for the overall plot. It could also attract the attention of new enemies or allies.
One possible reason for Natsu to return badly hurt in a fanfiction could be a very tough battle. Maybe he faced an extremely powerful enemy that was far beyond his initial expectations. For example, if he encountered a new type of dark mage with some unknown and dangerous magic. This could lead to him getting seriously injured and having to struggle to return.
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.
Well, he could be magically healed by one of his friends. Maybe Lucy uses her celestial magic to call a spirit with healing powers.
Well, if Naruto is badly injured in a Fairy Tail fanfiction, perhaps the Fairy Tail mages would try to heal him using their magic. They have various types of healing magic like Wendy's Sky Dragon Slayer magic which could be used to mend Naruto's wounds.
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.
There's also the possibility that it's not a dark force at all. It could be a well - meaning but misguided wizard who accidentally causes Harry to be badly hurt. For instance, during a training session that goes wrong or while trying to perform a difficult spell to protect Harry but ends up harming him instead.
Kiba might immediately take Naruto to the nearest medical ninja. In the Hidden Leaf Village, there are always medical ninjas on standby. Kiba's quick action could save Naruto from a more serious condition.
Sodapop could also try to tend to Ponyboy's injuries himself if they seem minor at first. He'd probably be full of worry and keep asking Ponyboy what happened. Maybe it was a fight with the Socs or an accident. Sodapop would be desperate to make sure Ponyboy is okay, and his protective instincts would kick in big time.
Next, Drake might take Meghan to the hospital if it's a physical injury. If it's an emotional hurt, he could just be there to listen and comfort her. Maybe they'll have a deep conversation that reveals their true feelings for each other.
Well, one possibility is that Dream could use his positive energy to heal Frisk. Since Dream is often depicted as a benevolent character, he might sense Frisk's pain and offer some sort of magical assistance. Maybe he has special powers that can soothe Frisk's wounds and give them the strength to continue on their journey.