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.
As of now, Naruto and Sasuke haven't died in the Boruto manga. Their characters continue to play significant roles and contribute to the story's development.