The main characters would surely be Naruto and Sasuke. Their final battle at the Valley of the End is a crucial part of the Naruto story. However, in a fanfic, characters like Itachi could also be brought in. His influence on Sasuke and his actions in the overall Naruto plot could be explored in relation to the Valley. And don't forget about Jiraiya. His teachings to Naruto could be revisited and related to Naruto's experiences in the Valley of the End.
One idea could be a what - if scenario where Naruto and Sasuke have a rematch at the Valley of the End years later, but this time they have to team up against a new, powerful enemy that threatens the entire ninja world. Another could be a story exploring the emotional aftermath for Naruto if he had actually killed Sasuke during their fight there. Maybe a fanfic where the Valley of the End becomes a place of peace and training for future generations of ninjas.
Perhaps in this fanfic, Naruto's death at the Valley of the End could lead to a new era of power struggles. The other ninja villages might try to take over the Leaf's territory. Meanwhile, Naruto's death could inspire a new generation of ninjas who want to uphold his ideals. They would have to fight against the odds to keep the peace that Naruto once fought for.
Sakura would be devastated. She has always seen Naruto as a dear friend and teammate. She would probably blame herself for not being strong enough to prevent his death. Sasuke, on the other hand, might feel a deep sense of loss and anger. He might go on a solitary journey to find out who was responsible.
Perhaps Naruto is betrayed by someone he trusts deeply. This person leads him into a trap at the Valley of the End. Naruto, being caught off - guard, is fatally wounded. His death in this fanfiction could be a tragic end to his journey of becoming a great ninja and a symbol of how even the most powerful can be brought down by treachery.
Sasuke could be responsible. Their relationship has always been complex. Maybe in this fanfiction, their rivalry escalates to a point where Sasuke accidentally kills Naruto during a fight at the Valley of the End.
A really good fanfiction idea could be that Naruto, after being saved at the Valley of the End, decides to take a different path in his training. Instead of focusing on his regular techniques, he delves deeper into the history of the Valley of the End. He discovers ancient secrets that were long forgotten, and these secrets not only help him become stronger but also help him to better understand the cycle of hatred that has been plaguing the ninja world for so long. He then uses this knowledge to try and bring about a more permanent peace.
I'm not sure of a specific title off - hand, but you can often find great ones on fanfiction websites. Look for ones with high ratings and lots of reviews. They usually are the better - written ones.
Madara Uchiha might be the main antagonist. He has grand plans for the ninja world. He could see Naruto as a threat to his vision and engineer a situation in the Valley of the End where Naruto is killed, either directly or by using Sasuke as a pawn.
Perhaps in this fanfiction, Naruto is fatally injured by a powerful enemy who has lured him to the Valley of the End. He's outnumbered and despite his best efforts, he succumbs to his wounds. His death then has a profound impact on all his friends and the entire ninja community, leading to a new era of unrest and a search for a new hero.