Naruto himself is always interesting. In this fanfiction, seeing him adapt to the new powers of Unlimited Blade Works and how it changes his fighting style would be fascinating. He might become more strategic and less reliant on brute force.
Sasuke could also be a very interesting character. Maybe he becomes jealous of Naruto's new power and tries to either obtain it for himself or prove that he doesn't need such a power to be strong. This could lead to some intense rivalries and battles between them.