It could be about Naruto choosing a solitary life after all the battles. Maybe he decides to live alone in the forest to find inner peace away from the chaos of the ninja world.
It could be a new and powerful dark ninja who thrives on isolation and wants to keep Naruto alone. He might see Naruto's solitude as a weakness to be exploited.
Well, Percy is obviously one of the main characters. His role in protecting is central to the whole concept. Annabeth is also likely to be a main character as she often stands by Percy and helps in his protection quests. Grover might also be there as Percy's loyal friend.
One great 'Percy Protection Forevermore' fanfiction could be the one where Percy is transported to a parallel universe and has to protect his friends from an unknown evil. In this story, his unique powers are put to the test in ways not seen in the original series.
Well, 'forevermore' gives a sense of eternity and 'ending story' implies it's about the conclusion of a tale. It might refer to a story whose ending is so significant that it becomes timeless. For example, a classic love story where the lovers' final union or separation is so poignant that it stays in people's minds forever.
One common theme is self - discovery. When Naruto is alone, he often has time to think about who he is and what he wants. Another theme is inner strength. He has to rely on his own abilities and not depend on others. For example, in many fanfictions, he overcomes great difficulties alone using his own skills.
He could meet an old enemy who has also changed after the war. This enemy might now be in a similar situation of being alone and they could form an unexpected bond as they understand each other's isolation.
He might meet an old sage who has been living in seclusion near the village. The sage could see Naruto's loneliness on his birthday and offer him some ancient ninja knowledge.
As I don't know the particular 'forevermore full story', it's hard to say. But usually in a story, there could be a protagonist who drives the story forward. It might be a young hero or heroine.