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.
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.
One of the main things that make it strong in fanfiction is the creative use of its basic properties. Writers can make Luffy use his stretching in ways that are not shown in the original manga or anime. He could stretch his internal organs to survive otherwise fatal blows. Or he could use his stretched body to create complex structures for defense or to launch multi - directional attacks. This freedom of creativity in fanfiction enhances the perceived strength of his Devil Fruit.
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.
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.
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.
Some fanfictions interpret it as having a connection to ancient deities. For example, Luffy's fruit could be related to a sun god, giving him the ability to emit intense heat or bright light. Others might see it as a key to unlocking hidden dimensions within the One Piece world, allowing Luffy to travel to places no one else can reach.