Perhaps Naruto has some sort of premonition about Sasuke's danger. He follows his gut feeling and finds Sasuke trapped by Haku. He then uses his Uzumaki bloodline power, something that might be enhanced in fanfiction, to disrupt Haku's chakra flow and save Sasuke.
In fanfiction, Naruto could have made a deal with some sort of mysterious force or character. This entity gives him a special ability or weapon just in time to save Sasuke from Haku. For example, he might be given a sword that can cut through Haku's ice with ease. He then uses this advantage to get to Sasuke and carry him to safety while fending off Haku's attacks.
In some fanfictions, Naruto might save a civilian who has been poisoned. He would use his knowledge of herbs and medical ninjutsu to find an antidote and save the civilian's life. There could also be a situation where a civilian is trapped in a burning building. Naruto would brave the fire, using his chakra to protect himself, and carry the civilian to safety. And sometimes, a civilian is being mind - controlled by an enemy, and Naruto has to break the control and save the civilian.
One scenario could be that Naruto is poisoned during a battle. Jiraiya, with his vast knowledge of herbs and medicine, finds an antidote in time to save Naruto. Another possibility is that Naruto is trapped in a genjutsu created by a powerful enemy, and Jiraiya uses his own genjutsu - breaking skills to free Naruto.
One possible scenario in this fanfiction could be that Naruto discovers Haku and Kimimaro in a dire situation. Haku, with his ice techniques, and Kimimaro, with his powerful bone abilities, are perhaps being overpowered. Naruto, being the hero he is, jumps into the fray. He may convince them to change their ways and join him in Konoha, as he once did with Sasuke. This would not only show Naruto's growth as a ninja but also his ability to bring out the best in others.
Another scenario could be that they are poisoned by a new, undetectable toxin. As they gradually weaken, they try to find a cure but run out of time. Their deaths would be a great loss, and it could lead to major changes in the political and social structure of the ninja world as everyone copes with their passing.
One possible scenario could be a power imbalance. Maybe Sasuke discovers some new, extremely powerful jutsu that Naruto has no defense against. For example, a jutsu that can nullify Naruto's chakra or attack his spirit directly. Another possibility is a miscommunication or a breakdown in their relationship. If Sasuke believes Naruto has betrayed him in some way, his anger could lead him to kill Naruto. However, this goes against the core of their bond in the original series.
One possible scenario could be that they sacrifice themselves to save the village from an ultimate evil. They might combine their powers in a final, desperate move that exhausts them completely, leading to their deaths.
Another scenario could be that Naruto and Sasuke discover a new power within themselves during the fight against Kaguya. This new power could be related to their friendship or their connection to the Sage of Six Paths. They then use this power to finally defeat Kaguya.
One possible scenario is that Naruto arrives in the Fairy Tail world just as a powerful dark guild is about to destroy Fairy Tail. He uses his speed and combat skills to take on the leaders of the dark guild and defeats them. Another scenario could be that Fairy Tail is facing a magical disaster, like a powerful spell gone wrong. Naruto, with his ability to adapt quickly, figures out a way to counter the spell using his own ninja powers.
One possible scenario is that Hinata and her team are on a mission in an unknown territory. They encounter a group of rogue ninjas. Hinata tries to defend her team but is outnumbered. Just as they are about to be defeated, Naruto shows up, having followed Hinata's chakra signature. He quickly takes down the rogues with his speed and strength.
Maybe in the fanfiction, the situation is more complex. Hinata is being held hostage by the Cloud in a heavily guarded location. Naruto infiltrates the place secretly. He uses his transformation jutsu to disguise himself as a Cloud ninja at first to get closer. Then, he reveals himself and uses his Sage Mode to take on all the guards. He finally rescues Hinata and they escape together.