Well, perhaps Percy comes back to find his old friends have changed a great deal. Maybe some have new powers or positions in their world. He could be faced with a new threat that only he can help to defeat because of his unique past experiences.
Well, perhaps Percy comes back with new and amazing powers. He might be like a hero who has seen many things during those years away. He could use his powers to save his old friends from a big danger that they can't handle on their own.
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.
Well, he might disappear because of a powerful curse from an ancient enemy. Maybe he gets trapped in a different dimension. When he returns, he could have new powers or knowledge. For example, he might have learned some secret magic from the other dimension that helps him defeat a new threat in the Percy Jackson world.
His old friends like Annabeth could be main characters. She might have been leading the group in Percy's absence and now has to adjust to his return. Grover could also be there, still being Percy's loyal friend and perhaps having some new knowledge about the world of magic and nature.
Well, it could involve Percy coming back with new powers. Maybe he's more in tune with the underworld's magic. He might have made some sort of deal in Tartarus and now has to deal with the consequences in the mortal world.
In addition to Annabeth and his family, he might interact with other demigods he knew before. They could be in awe of his new godlike state. There could also be some characters from the godly realm who are interested in Percy's return. They might want to use his power for their own purposes or they could be there to guide him on how to fully control his newfound abilities.
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.
One possibility could be that Percy returns with new powers. Maybe he has some connection to the Underworld now that he's come back from the dead. He might be able to control the dead or has some knowledge about the afterlife that he uses to help his friends in battles against new and more powerful enemies.
One possible plotline could be Percy having nightmares about his time in Tartarus. He might struggle with PTSD, which could affect his relationships with his friends and family at Camp Half - Blood. Another could be that he brings back some sort of dark power or knowledge from Tartarus that he has to figure out how to control or use for good. Maybe he discovers a way to close the entrance to Tartarus permanently so that no more monsters can come out and endanger the demigod world.
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.