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.
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.
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.
If Fem Luffy had a different Devil Fruit, she might not have the stretchy body that makes her so recognizable. For example, if she had a Devil Fruit related to plants, she could grow vines from her body to grab enemies or create barriers. This would make her more of a strategic fighter, relying on the environment she can create rather than her physical strength alone.
If Luffy had a Devil Fruit that made him invisible, it could make him a bit more mischievous. He might start playing pranks on his crewmates more often. But his core values of loyalty and determination would remain. He would still use this power for good in battles, sneaking up on enemies and catching them off - guard. The new power would just add a different flavor to his already vibrant personality.