One idea could be that Naruto and Sasuke are somehow transported to the world of Kill la Kill. They might get involved in the fight against the Life Fibers, with their ninja skills being a unique addition. Maybe Naruto's Rasengan could be used to disrupt the Life Fibers in a new way.
I haven't actually read this particular fanfic in detail. But if it's about changing fate and killing Sasuke, it could be a really interesting take on the Naruto universe. It might explore how different characters' fates are altered when Sasuke is killed, and perhaps show new power dynamics and relationships emerging.
In some cases, Sasuke could be angry. He might see Naruto's request as a sign of weakness or as Naruto giving up too easily. He could lash out at Naruto, telling him to fight instead of asking for death. However, he would still be deeply troubled by the request and would probably struggle with it for a long time, constantly thinking about whether or not he should consider it.
Naruto might be very friendly and curious about the characters from Kill la Kill. He could try to befriend Ryuko immediately and ask her about her cool outfit and powers. Sasuke, on the other hand, would be more cautious and might see them as potential threats at first.
One reason could be Sasuke's thirst for power. If he believes Naruto is the only obstacle in his path to ultimate power, he might be driven to kill him. In some fanfictions, Sasuke may be influenced by a dark entity that twists his mind into thinking Naruto is a threat that needs to be eliminated.
One possible reason could be his thirst for power. He might see Naruto as an obstacle in his pursuit of greater strength. Maybe he believes that by eliminating Naruto, he can prove himself to be the stronger one and gain more recognition in the ninja world.
It's unique because it goes against the normal narrative of the Naruto series. Usually, Sasuke is a central character, but in this fanfic, his death is a central plot point. This could lead to unique character developments for other characters.
One possible motive could be if Sasuke was on a path of destruction that endangered the entire ninja world. Naruto, as the protector of peace, might see killing Sasuke as the only way to stop him. For instance, if Sasuke was about to unleash a powerful and uncontrollable jutsu that would wipe out many innocent lives.
Perhaps in the fanfic, Sasuke was being pushed to his limits. He was so intent on protecting the village from a powerful threat that he didn't realize Sakura was in a dangerous position. His Sharingan might have malfunctioned in some way, causing him to misinterpret Sakura's movements as hostile. Or, there could be a situation where an enemy was using Sakura as a shield, and Sasuke's attempt to strike the enemy ended up harming Sakura instead, all in a split - second decision.
Perhaps Sasuke is possessed by some extremely evil entity. And Kakashi, in an attempt to stop the entity from causing more destruction, decides to kill Sasuke. It could also be that Sasuke discovers some dark secret about Kakashi that Kakashi doesn't want revealed, and in a moment of panic and self - preservation, he kills Sasuke.