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.
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.
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.
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.
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.
The plot of this novel could center around the great mage's return after 4000 years. When he comes back, he might find that the place he once knew is almost unrecognizable. His return could be a catalyst for many events. He may have to use his ancient magic to deal with modern - day problems. There could be a mystery about why he disappeared in the first place and what led to his return. He may also have to face the consequences of the passage of time, like the decay of old magic sites or the rise of new dark forces that he has to combat with his age - old wisdom and power.
I'm not sure specifically as I haven't read it in detail. But generally, it might be about a great mage coming back after a long period of 4000 years, perhaps to a world that has changed a great deal and facing new challenges, enemies or reuniting with old comrades.
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.
Well, from the title, it could be a story full of magic and adventure. The great mage returning after such a long time, 4000 years, might find the world completely different. There could be new magical systems, new races, and new threats. He or she might have to use their old - school magic skills to adapt to the new world and face the challenges that come their way.