I recommend the two novels, One Piece: I'm a Big Boss and Naruto: The Demon's Law, to you. The main character of One Piece I Am a Big Boss became the West Ocean mafia in the world of One Piece. He ate the fruit of the Demon's Eye and became the world's big boss. Meanwhile, the main character of Naruto Demon Law, Medicine Master Yu, awakened the Demon Fruit in Naruto World, and the human puppets he created also had the ability of the Demon Fruit. I hope you like my recommendation.
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.
Well, Naruto would probably become more confident. With all those devil fruit powers, he could be even more fearless in battles. He might start to rely less on his original ninja skills and more on the unique abilities from the devil fruits, which could change his fighting style drastically.
The powers of devil fruits can be integrated by having characters discover them in a strange new land that has a connection to both the Naruto and One Piece universes. For example, a portal could open up and Naruto and his friends find a devil fruit tree. They then have to figure out how these new powers work within the context of their own ninja abilities. Maybe the devil fruit powers can be combined with chakra to create even more powerful attacks.
Another possibility is that the character has some sort of technological device. In a more sci - fi inspired fanfiction, a super - advanced gadget could be created that can scan and reproduce the energy signatures of Devil Fruit powers. This device might have been created by a brilliant scientist who was studying the Devil Fruits. The character then gets their hands on this device and uses it to copy the powers. However, using this device might come with its own set of risks, like overloading the body with too much power or attracting unwanted attention from powerful villains.
One consequence could be that the character becomes a target. Since having the ability to copy Devil Fruit powers is extremely powerful, many villains would want to either control or eliminate the character. This would put the character in constant danger.
For characters like Wendy, if she got a Devil Fruit power, she'd need to be careful not to overexert herself. Since she's a healer, she might get a Paramecia - type Devil Fruit that gives her the ability to heal more effectively. But she'd have to learn how to balance it with her Sky Dragon Slayer Magic. And characters like Elfman would need to find a way to use their Devil Fruit power in harmony with their Take - Over Magic. If he got a Zoan - type Devil Fruit, he'd need to figure out how to transform between his normal form, his Take - Over forms, and his Devil Fruit form without getting confused or losing control.