Perhaps Naruto is transported to the world of Unlimited Blade Works and has to learn to use its powers to get back home. Along the way, he could meet characters from that world and form new alliances. He might also bring some of the concepts from Unlimited Blade Works back to his own world and revolutionize the way ninjas fight.
Well, there might be some out there. You could try searching on fanfiction websites like FanFiction.net or Archive of Our Own. Just type in the keywords 'Naruto' and 'Unlimited Blade Works' and see what comes up.
Perhaps there is a wish - granting entity in the fanfiction. Naruto, in his quest to protect his village or friends, makes a wish to be able to use any power necessary. The entity grants him the ability to use Unlimited Blade Works. He then has to train himself to master it, which would be an interesting journey as he tries to balance his ninja way with this new, very different power.
One consequence could be that his enemies are completely caught off - guard. Naruto's new ability to summon a vast number of weapons at will would change the dynamics of battles. Enemies used to his ninja attacks would now have to deal with a whole new type of assault.
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.
The plotlines in such fanfiction can vary greatly. Some might focus on alternate universes where the events of 'Unlimited Blade Works' take a different turn due to the actions of the OC. For instance, the OC could change the outcome of a battle, which then has a domino effect on the rest of the story. It could also explore new concepts that were not fully explored in the original work, like a different type of magic or a new backstory for a familiar location.
Perhaps in this fanfiction, Naruto enters the Unlimited Blade Works realm and has to face his own inner demons represented as clones within that world. This would be a great way to explore his character deeper as he battles his fears and insecurities.
A unique feature is the combination of two different universes. Naruto's ninja abilities like shadow clones and rasengan blend with Unlimited Blade Works' concept of noble phantasms.
Power-ups and new abilities are also a theme. Writers might give a character a new skill or a stronger version of their existing powers. For example, Shirou could discover a new way to project his weapons that gives him an edge in battle. And the theme of moral dilemmas often appears. Since the original work has complex moral issues, fanfiction can expand on these. Characters may face even tougher choices about using their powers and the consequences of their actions.
The plot is crucial. It could be an interesting alternate universe where Issei gets involved in the events of Unlimited Blade Works in a different way. Maybe he has a different power - up or forms new alliances. A good fanfiction also has a good flow, not too rushed or too slow - paced, like building up the tension before a big battle scene in a way that keeps the reader hooked.