Many fanfiction writers will show Naruto's ninja skills evolving in the Forest of Death. He could learn from the environment, like using the trees and foliage to his advantage in new ways. Maybe he figures out how to use the natural chakra in the forest to boost his own power, which is a significant change that would set him apart from his previous self.
Fanfiction often depicts changes in Naruto's personality. In the Forest of Death, he may face situations that force him to be more serious. He might lose some of his naivete and become more worldly. For example, if he witnesses the cruel actions of other ninjas, he could become more cynical or, on the other hand, more determined to uphold his ideals of friendship and peace.
Some writers might show Naruto's physical changes. He could gain more strength or speed after facing tough opponents in the forest. For instance, if he has to constantly run away from a powerful enemy, his leg muscles could develop more, making him faster in future battles.