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.
Gray would be one of the main characters to interact with Natsu. Their friendly rivalry is well - known in Fairy Tail. Makarov, as the guild master, would also have an important role in interacting with Natsu. He might give Natsu some important tasks or just have a grandfatherly chat with him about his return.
His close friends like Lucy would be extremely happy. They might cry tears of joy and immediately hug him. Gray, on the other hand, might act cool at first but then show his true happiness. Erza would probably be the most composed but still show her delight through a warm smile.
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.
Maybe Natsu comes back with new powers. He might have to face old enemies that have grown stronger in his absence. Also, his relationships with his friends could have changed, and he'd have to work on rebuilding those bonds.
One possible plot could be that Natsu returns with newfound powers. He might use these powers to help rebuild the world after the war, and also to reunite with his friends. Maybe he has to face some remnants of the enemy forces that still lurk in the shadows.
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.
One interesting plotline could be Natsu coming back with new and powerful magic. For example, he might have learned some ancient spells during his time away. Another could be his reunions with the various members of Fairy Tail. He would have emotional moments with Lucy, Happy, and the others, reminiscing about old times and looking forward to new adventures together.
Maybe it's an old enemy who survived the war and has grown stronger. They hold a grudge against Natsu and his friends and are now seeking revenge. For example, if there was an enemy that was thought to be defeated before the war but managed to survive and is now back with a vengeance.
Obviously, Natsu himself is the central character. His friends like Lucy, Gray, and Erza are likely to be involved. They would be the ones to welcome him back and also be part of his new adventures or challenges that come after his return.