There might be a fanfiction where a devil in DxD accidentally eats a Paramecia devil fruit. This could lead to all sorts of comical and also strategic situations. Say it was the Gomu Gomu no Mi (Rubber - Rubber Fruit). The devil's normal abilities combined with the stretchy powers of the fruit would make for some really unique fight scenes and interactions with other characters in the DxD world.
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.
Rias is also a popular choice. If she were to have a devil fruit power in fanfiction, it could completely change the power dynamics in the story. For example, if she had the Ope Ope no Mi (Operation - Operation Fruit), she could perform incredible surgical - like attacks and also heal her allies. Her leadership role in DxD combined with such a powerful devil fruit would make for very interesting storylines.
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.
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.
One interesting idea could be that Luffy has to rely solely on his Haki and physical training, making him even more of a badass in combat. He might develop unique fighting styles that we haven't seen in the original story where he had devil fruit powers.
One interesting idea could be that Luffy's dragon devil fruit gives him the ability to control different types of weather related to dragons, like summoning thunderstorms or creating strong winds. Another could be that he gains the power to transform into a dragon - like form with enhanced strength and speed.
Some fanfictions explore the idea of devil fruits being passed down in a different way. Instead of just randomly appearing in the world, they could be part of a family heritage. Also, there are fanfictions where the side - effects of devil fruits are more extreme. For instance, instead of just not being able to swim, the user of a devil fruit might have a more complex weakness related to water, like their powers weakening even when it's just humid.
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 exploring a new power that the third devil discovers. For example, a unique ability to manipulate time on a small scale during battles, which would add a whole new dimension to the fight scenes. Another idea is to focus on the third devil's relationships with other characters in a different way. Maybe the third devil forms an unexpected alliance with a group that was previously an enemy, leading to some really interesting character development and plot twists.
One idea could be a new devil character arriving at Kuoh Academy with a unique power that challenges the existing power dynamics among the devils there. For example, a devil who can manipulate time in small increments during battles, which would make for some really intense and strategic fight scenes.