However, in the world of One Piece, it's been said that eating two devil fruits would cause a person to explode. But in fanfiction, this rule might be bent. Another consequence could be that Luffy's personality changes. The new devil fruit power might come with some sort of consciousness or influence that alters his decision - making or how he views the world. He might become more aggressive or more cautious depending on the nature of the new power.
In such a fanfiction, perhaps Luffy's character would change drastically. The dark dark fruit might give him new powers like controlling shadows more intensely than Blackbeard. He could use it to create shadow clones of his crew for strategic battles, adding a new and darker dimension to his fighting style.
One interesting plotline could be Luffy struggling to control the new powers at first. Since Logia fruits give the user the ability to become an element, Luffy might accidentally turn into his Logia form at inopportune times, causing chaos among the crew. For example, if his new power is related to fire, he could set the sails on fire by accident while trying to learn how to use it.
The World Government could become the main antagonist. They would see Luffy's new power as a threat, especially if he starts using it in ways that oppose their rule. They might send out their strongest agents, like the Admirals, to take Luffy down. The Admirals could have developed new techniques to deal with the dark dark fruit powers, making them a formidable foe for Luffy.
Cheng Jushu ate the fruit because Teng Zijing's son gave him a pear. This kind gesture touched Cheng Jushu very much. In his entire life, other than Teng Zijing's son, no one was willing to make friends with him or be friendly to him. This precious friendship changed Cheng Jushu, making him willing to give up his life for Teng Zijing's son.
Cheng Jushu ate the fruit because Teng Zijing's son gave him a pear. This kind gesture touched Cheng Jushu very much. In his entire life, other than Teng Zijing's son, no one was willing to make friends with him or be friendly to him. This precious friendship changed Cheng Jushu, making him willing to give up his life for Teng Zijing's son.
If Luffy ate a different devil fruit, his entire power set and character would change drastically. For example, if he ate a Logia type like the Smoke - Smoke Fruit, his fighting style would be more about evasion and stealth. He could turn into smoke to avoid attacks, and his stretching abilities would be gone. His crew might also have to adjust their battle strategies around his new powers.
The story's pacing would also likely change. With the new power, Luffy could reach his goals more quickly. For example, in the search for the One Piece, his enhanced abilities might allow him to access areas or defeat obstacles that were previously insurmountable. This could mean that parts of the story that were about building up strength and exploring the world in the original would be rushed or skipped altogether in the fanfiction.
In some fanfictions, Luffy's Devil Fruit is indeed considered the strongest. His Gum - Gum Fruit gives him unique abilities like stretching his body in incredible ways. In fan - created stories, these abilities can be enhanced even further, making him almost unbeatable.
Imagine Luffy ate a devil fruit that gave him the power to control minds. This would be a very different power compared to his current Gum - Gum abilities. His crew would have to trust him completely not to abuse this power. There could be ethical discussions within the crew. Also, in terms of leadership, Luffy might have an easier time getting others to follow his plans, but it could also create some tension if the crew felt he was relying too much on his mind - control powers instead of his natural leadership qualities.