In this kind of fanfiction, Sasuke could die from a sneak attack. He might be so focused on his main target during the Wave mission that he doesn't notice another hidden threat. And this hidden enemy could strike him when he least expects it, leading to his death. For example, an assassin could be hired to specifically target Sasuke and use some sort of poison or a deadly jutsu that he can't defend against in time.
Sasuke's determination serves as a major driving force. It might force him to use his skills and powers in new and creative ways. He may have to face his own limitations and grow stronger in order to keep Naruto alive. This growth could be both in terms of his ninja abilities and his emotional maturity. As he tries to save Naruto, he could also be forced to confront his past actions and make amends, which would be a big part of the plot development.
Well, it would have a huge impact. For Naruto, losing his sensei would be like losing a father figure. He'd be deeply saddened and might feel a bit lost in his ninja journey. The other characters like Shikamaru and Kiba would also be affected. They might start to question the safety of being a ninja. And the village as a whole would be in mourning. This could lead to a change in the village's dynamics, with more caution in sending ninjas on dangerous missions.