In most One Piece canon, a person can't eat multiple devil fruits due to the risk of exploding. However, in fanfiction, the rules can be bent. Some fanfictions might allow Luffy to eat multiple devil fruits by creating special circumstances or new powers that counteract the normal limitations. For example, a fanfiction could introduce a powerful magic or a unique ability that enables Luffy to safely consume and control the powers of more than one devil fruit.
If Luffy ate multiple devil fruits in fanfiction, it would likely make him extremely powerful. His existing Gum - Gum powers are already quite strong, but adding more devil fruit powers would be insane. He could have abilities like teleportation from one devil fruit and the ability to turn invisible from another. This would completely change the power dynamics in the story. Enemies that were once a challenge would become much easier to defeat. However, it could also bring new challenges. Maybe the new powers are difficult to control at first, and Luffy has to struggle to master them. Or perhaps there are side - effects to having multiple devil fruits that he has to deal with.
He could eat the Pika Pika no Mi (Glint - Glint Fruit) which gives the power of light, allowing him to move at the speed of light and create powerful light - based attacks. And perhaps the Yami Yami no Mi (Darkness - Darkness Fruit) which can absorb other attacks and create a gravitational pull. Combining the powers of light and darkness would be quite a unique and powerful combination for Naruto in this fanfiction.
Well, he might gain new powers which could make him more confident. For example, if one of the devil fruits gives him the power of flight, he would feel more powerful and it could change his approach to battles. He might become a bit more adventurous too, as he explores the limits of his new abilities.
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.
One potential combination could be if Naruto ate the Ope - Ope Fruit (giving surgical - like powers) and the Inu - Inu Fruit (Dog). He could use the Ope - Ope powers to manipulate his body parts while in his Dog form, perhaps making his attacks more precise or his defenses stronger. This would also open up a whole new set of strategies for him in battles. It's all about how the fanfiction author would choose to blend these very different power sets and how Naruto would learn to use them effectively.
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.
Another idea is a Devil Fruit that gives Luffy the ability to transform into different animals. He could turn into a powerful eagle to fly over islands and scout for danger or transform into a strong bear to have enhanced strength and endurance in close - combat battles. This would change his fighting style completely and make for some really interesting interactions with his crew and opponents.
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.
Another fanfiction concept could involve Naruto being sent to the One Piece world through some kind of strange ninja jutsu gone wrong. There, he eats a Logia Devil Fruit that allows him to turn into water. He teams up with the Straw Hat Pirates and uses his water - based powers, along with his ninja knowledge of chakra control, to help them in their adventures. For example, he could use his water form to put out fires on the ship or create water - based defenses against enemies.