Well, a cool concept in Naruto Re Zero fanfiction might be the idea of Sasuke in the Re Zero world. He could be seen as a very powerful and mysterious figure, with his Sharingan being a great asset in the world full of magic and danger. Also, the idea of Naruto befriending Subaru and learning from each other's experiences of facing difficulties over and over again can be really interesting.
In Naruto Re Zero fanfiction, a fascinating concept could be the application of chakra in the Re Zero setting. Since chakra is a form of energy in Naruto, it could interact in unique ways with the mana in Re Zero. Maybe a character could combine chakra - based jutsus with Re Zero's magic spells. Also, the story could explore how the different power hierarchies in Naruto would compare to those in Re Zero, like how a jonin - level ninja would fare against a powerful mage in the Re Zero world.
In Naruto re monster fanfiction, a cool concept could be the creation of new monster species within the Naruto universe. Maybe there are ancient monsters that were sealed away long ago and are now being released, causing chaos. The characters would then have to adapt to fight these new threats.
Another idea is to write a fanfiction where the roles are reversed. Make Emilia the one with a special power and Subaru as the more 'normal' character who has to support her. It could be fascinating to see how Emilia would handle the burden of a power like Return by Death and how Subaru would adapt to being in a more supportive role.
In 'naruto familiar of zero fanfic', there could be an exploration of different cultures. The shinobi culture from Naruto and the magical culture of 'Familiar of Zero' might clash or blend. This can result in interesting storylines, such as how the characters from both worlds view concepts like honor, loyalty, and friendship in the context of this new combined world.
Well, one interesting concept could be the origin of the Zero Tails. Maybe it's a new form of chakra manifestation that was previously unknown in the Naruto universe. It could also involve unique abilities that the Zero Tails has, like the power to manipulate a different type of energy compared to the regular tailed beasts.
One interesting concept could be alternative timelines. Writers might explore what would happen if certain events in the original 'zero no tsukaima' series occurred differently. For example, if Louise had chosen a different familiar. Another concept could be character development beyond what was shown in the anime or manga. Maybe Saito becoming even more powerful in magic in ways not originally depicted. And also, exploring different relationships between the characters, like a deeper friendship between Louise and her classmates that was only hinted at in the original story.
One interesting story could be about Subaru using his 'Return by Death' ability in a new and unexpected way within the world of watches. Maybe he discovers a watch that has some sort of magical power related to time, and his ability interacts with it in a strange manner.
Another possibility is a crossover with 'My Hero Academia'. Subaru could be a student at U.A. High School. His Return by Death power would be like a quirk. He might struggle at first to control it and fit in, but eventually use it to save his classmates from villains, in a way that surprises everyone with his growth and resourcefulness.
In some 'Re Zero Gamer Fanfiction', the concept of leveling up could be introduced. Subaru discovers that he can gain experience points not just from battles but also from making important decisions or helping the people of the world. This changes his approach to the world and creates a whole new narrative arc as he tries to level up as fast as he can to become more powerful.
One interesting 'Re Zero Reaction Fanfiction' could be the ones where the characters react to modern technology. For example, Subaru being introduced to a smartphone and his hilarious reactions to all the apps and functions. Another could be the characters' reactions to different cuisines from our world. Emilia trying pizza for the first time and her delight in the new taste.
You could create an OC who is a master of illusions. This character could use their powers to deceive both enemies and allies, and their backstory could involve being an outcast from their own illusionist clan because they refused to use their powers for evil. In the Re:Zero world, they could help Subaru by creating diversions during battles or by using illusions to uncover hidden truths.