Well, one possibility could be that Naruto returns with new powers. Maybe he has some kind of connection to an ancient force during his time 'dead' and now he has to face new threats that emerged while he was gone. For example, a new evil organization could have risen, and his return gives hope to the Hidden Leaf Village again.
A new dark entity could be the main antagonist. This entity might have been awakened by Naruto's return from the dead. It could be something that was sealed away long ago and Naruto's return breaks the seal. The entity would then target Naruto and his loved ones to prevent him from getting in its way of ruling the ninja world.
One possible plot could be that Percy returns with new powers. Maybe he was resurrected by some ancient magic or a powerful deity who has a plan for him. He then has to deal with the shock and awe of his friends and also face new enemies who are either threatened by his return or want to use his newfound powers for their own gain.
It could be a jealous demigod who thought that with Percy dead, they would be the most powerful or important one. Now that Percy is back, they see him as a threat to their status and plan to get rid of him again.
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.
Maybe it's a new character who is jealous of Percy's status among the demigods. This new antagonist could be a powerful demigod from a different camp who has heard of Percy's feats and doesn't want him to steal the spotlight again. So, they try to use Percy's return from the dead as a way to turn the other demigods against him.
In the fanfiction, Naruto's return from training could also lead to a change in his relationships. His friends might be in awe of his new strength and some might even feel a bit of distance as he has become so much more powerful. Also, he could use his new abilities to protect the village in a more effective way and gain more respect from the villagers.
It varies from one fanfiction to another. Some fanfictions might depict Naruto as dead to introduce a new set of challenges for the remaining characters. For example, if Naruto dies, Konoha might be left vulnerable, and characters like Sasuke or Sakura would have to step up in different ways. But there are also plenty of fanfictions where Naruto remains alive and continues his journey, perhaps facing new and even more dangerous threats than in the original series.
Naruto's character development in this fanfiction might involve him becoming more of a leader. With his new skills from training, he can inspire others and lead by example. He may also be more patient, having learned the value of hard work and perseverance during his training.
The fanfiction may change the pacing. In the original Naruto, there are specific story arcs and build - ups. However, 'naruto returns changed' might speed up or slow down certain parts. Maybe his return is rushed in the fanfiction, or there are more flashbacks and introspection compared to the original.