Well, the OC could be in a situation where their absorption power goes out of control. Maybe they accidentally absorb too much energy from a powerful source and start to lose their sense of self. Their friends in the pirate crew or wherever they are have to find a way to reverse the process. They search for a special healer or a rare item that can help. In the meantime, the OC is a danger to themselves and others as their uncontrolled absorption power causes chaos around them.
First, give the OC a unique backstory. Maybe they got the Absorption Devil Fruit by accident, like finding it in a hidden chest on an abandoned island. For their appearance, you could give them markings on their body that glow when they use their power. In terms of personality, make them curious. Since they can absorb different things, they would always be eager to test the limits of their power. For example, they might be the type to immediately try to absorb a strange new substance they come across.
Well, an interesting concept for a One Piece OC Devil Fruit in fanfiction could be a fruit that enables the user to control and generate different types of light. They could use it for offensive purposes, like creating blinding flashes or concentrated laser - like beams. Or for defensive, by creating a shield of solid light. Also, a Devil Fruit that gives the power to communicate with all sea creatures, not just the ones like Luffy can with his limited ability, would be really cool. This would open up a whole new world of possibilities for adventures and interactions in the One Piece universe.
One idea could be that the OC uses the Magma Devil Fruit to create new forms of terrains. For example, they could form volcanic islands in the middle of the ocean, which would change the geography and power dynamics in the story's world. They might also use the magma to create defensive barriers that are almost impossible to penetrate, like a ring of molten rock around a city they are protecting.
First, think about the OC's backstory. Maybe they got the Magma Devil Fruit by accident, like finding it on a deserted island. Then, decide on their goals. Are they looking for power? Or do they want to use the power for good? Next, introduce some antagonists. It could be other pirates who want the fruit for themselves. As for the plot development, start with the OC learning to control their new power. They could face difficulties like accidentally setting things on fire at first.
One idea could be that the OC is a traveling artist who stumbles upon Rapunzel's tower. Rapunzel is intrigued by the OC's art and they start to create art together, using the long hair as an unconventional paintbrush. This could lead to a friendship where they both learn about the outside world through each other's eyes.
One plot idea could be Mildred getting involved in a time - traveling adventure. Maybe she accidentally gets sent to a different era in the Futurama universe and has to find her way back while dealing with the strange characters and situations of that time. For example, she could end up in the pre - apocalyptic world and have to team up with some of the early inventors to fix the time - travel device.
One idea could be that Naruto somehow obtains a Dragon Devil Fruit. He then gains the abilities like flying, breathing fire or some other dragon - like powers. This would completely change the dynamics of his battles in the Naruto universe. He could use his newfound flight to gain a strategic advantage over his opponents and his fire - breathing could be a powerful offensive move.
Another possibility is a Devil Fruit that gives her the power of telepathy. Fem Luffy could then communicate with her crew without speaking, which would be very useful in stealth missions. She could also use it to read the minds of her enemies, getting an edge in battles. This would completely change the dynamics of her relationships with other characters in the story as well.
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.
Maybe an OC devil who was once a human and was turned into a devil against their will. They could struggle with their new identity while still having some human emotions, which would create complex relationships with the Hellsing characters.