One interesting crossover could be Naruto having the power of the Gomu Gomu no Mi (Rubber - Rubber Fruit). He could use his ninja skills in combination with the stretching abilities. For example, he could stretch his arm to surprise - attack enemies from a distance while still being able to perform his signature Rasengan. Another could be a character like Sasuke with the power of the Yami Yami no Mi (Dark - Dark Fruit), which would enhance his already dark and brooding nature and give him new powers to control darkness and absorb attacks.
One interesting concept could be a character with a Devil Fruit power that allows them to control elements, like Luffy's Gum - Gum Fruit but in the Naruto universe. For example, they could use it in combination with ninjutsu. Another concept might be a bloodline that is related to a Devil Fruit, where the power is passed down generations and has unique interactions with chakra. Maybe a bloodline that gives the ability to transform into a powerful beast - like form, similar to Zoan - type Devil Fruits in One Piece.
One idea could be a story where Naruto somehow gets a devil fruit that gives him the power to control water. He could use it in his battles in the Naruto world, like creating huge water jutsus with the unique properties of the devil fruit power. Another idea is to have a crossover where characters from One Piece come to the Naruto world and Naruto has to deal with the chaos that the devil fruits bring along.
One interesting concept could be the creation of new and unique devil fruits. For example, a devil fruit that allows the user to control time in a very specific way, like only being able to slow down time for inanimate objects. Another concept might be the exploration of how different characters from the original series would react if they ate a particular devil fruit. It could lead to completely new storylines and character developments.
Well, a really cool crossover might be Gray with a Zoan - type Devil Fruit that turns him into an ice - related creature. It could be something like an ice - dragon - like form which would blend well with his ice - make magic. Also, Erza could have a Devil Fruit that gives her the power to change her armors instantly just by thinking, a kind of Paramecia power related to her equipment. It would make her battles even more efficient and unpredictable.
If Naruto ate a Paramecia Devil Fruit, his physical abilities might change drastically. For example, if it was a rubber - like Paramecia fruit, his already acrobatic fighting style could become even more so. He could stretch his limbs to attack from unexpected angles, much like Luffy in One Piece. This would give him an edge in close - combat situations, and his opponents would have a hard time predicting his moves.
If Naruto had the Fire Devil Fruit, his combat style would change drastically. He could combine his existing ninja skills with the power of the fruit. For example, he might be able to infuse his Rasengan with fire, making it even more powerful. Also, his mobility could be enhanced as he could use fire to propel himself in new ways. This would make him a more formidable opponent in battles against strong foes like Sasuke or Pain.
The most interesting aspect is the combination of powers. Naruto characters already have their own unique ninja abilities, and when you add devil fruits from One Piece, it creates a whole new range of power combinations. For example, a character with a Logia - type devil fruit could become nearly invincible in Naruto's world, which would completely change the dynamics of battles.
One interesting combination could be Naruto's Rasengan with the power of the Pika Pika no Mi (Glint - Glint Fruit). He could create a Rasengan that emits intense light, blinding his opponents as he attacks. Another could be using his Shadow Clone Jutsu with the Mera Mera no Mi (Flame - Flame Fruit). His clones could all be on fire, creating a very intimidating and powerful offensive.
A possible plot could be that the devil fruit that Shiki has is a Logia - type devil fruit which allows him to turn into a certain substance. In the Naruto world, this confounds Naruto and his friends as they've never faced such an ability before. Shiki uses this power to infiltrate villages undetected. Naruto starts to study the nature of the devil fruit's power with the help of some of the more knowledgeable characters in the series, like Jiraiya (if he's still alive in this fanfiction). They eventually find a weakness in Shiki's power, which lies in a certain type of chakra that can disrupt the devil fruit's transformation process.
Another interesting concept is to cross the devil fruits with the world of Harry Potter. Imagine a character with a devil fruit ability like the ability to turn into an animal (zoan type) trying to blend in at Hogwarts. They might have to hide their powers from the wizards while also using them to their advantage in the magical world, like using their animal form to explore hidden areas of the school.