You can make the 'Naruto and Cinder Fall' fanfiction unique by changing the setting. Instead of the normal settings of their respective worlds, put them in a completely new and strange place, like a dimension where time and space work differently. Have Naruto and Cinder rely on each other's skills in this new environment. For instance, Naruto's sensory abilities might be crucial in detecting hidden dangers, while Cinder's magic could be used to create shelter or find food. This new and unfamiliar setting would force them to adapt and develop new strategies together, which would make the fanfiction stand out.
To make it unique, you could give Cinder some of Naruto's ninja abilities like the ability to walk on walls or use substitution jutsu. This would be a really unexpected twist for readers who know Cinder from her original story.