Sure, in fanfiction there are no real limits. Luffy might survive eating another Devil Fruit if the author decides to create a new mechanism. For example, there could be a magical item or a rare fruit that cancels out the negative effects of consuming multiple Devil Fruits. Or perhaps his strong willpower and unique body constitution, which already allowed him to adapt to the rubber Devil Fruit, could also help him handle a second one.
If Luffy ate another Devil Fruit in fanfiction, there are numerous possibilities. Firstly, it might go against the established rules in the original story about not being able to consume more than one Devil Fruit without dire consequences. However, in fanfiction, the rules can be bent. He could potentially have powers that interact in unique ways, like his rubber body being able to absorb the energy from the second Devil Fruit's power. This could give him new ways to attack and defend. His friends might be worried about the potential negative effects on his health, but also excited to see what new abilities he'll develop. And the villains would surely be even more afraid of him, as his power level could skyrocket in unforeseen manners.
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.
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.
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.
It might make his crew members view him in a new light. They could be in awe of his new dragon - like powers and be more protective of him as they know he has a very powerful and perhaps more dangerous ability now. For example, Nami might be more cautious about Luffy getting into fights as his dragon powers could attract more powerful enemies.
The uniqueness of 'no devil fruit Luffy fanfiction' lies in the fact that it challenges the traditional view of Luffy. Without his devil fruit powers, Luffy has to rely on his other qualities like his determination, his Haki, and his physical combat skills. It's interesting to see how he overcomes challenges in a different way.
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.
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.