His fighting style would become more elusive. If it was a Logia like fire, he could just let attacks pass through him as he turns into flames. He could also use his element to surround his opponents and trap them, which is a very different approach from his usual straightforward punches.
Well, Naruto eating a Logia devil fruit would be really interesting. He is known for his ninja skills in the Naruto universe. But with a devil fruit, things would change. Logia fruits are special in the One Piece world. If Naruto had one, say a Logia fruit that gives the power of lightning. He could use his new lightning powers to enhance his speed even more. His movements could be as fast as lightning strikes, and he could also use the lightning to attack his enemies from a distance. This would be a whole new way of fighting for him, different from relying solely on his ninjutsu and taijutsu.
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.
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.
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 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.
In fanfiction, if Luffy ate the Flame - Flame Fruit, he would gain the power of fire. His attacks could be infused with flames, making his Gum - Gum Pistol even more powerful. He could also use fire to defend himself, like creating a wall of flames around himself.
Naruto's personality might become more confident. Having the power of a Logia devil fruit would make him feel even more capable in battles. He might be a bit more reckless too, as he tests out his new powers.
A Logia of 'Crystal' could be really interesting for a male OC in a One Piece fanfiction. He could transform into various crystalline structures. He could use these to refract light and create illusions, or he could form sharp crystal shards to attack. Also, his body in crystal form could be extremely durable, making him a tough opponent. In battles, he could even create crystal mazes to trap his enemies.
There's also 'Logia Oc: The Hidden Power'. In this fanfiction, the oc has a logia power that is initially misunderstood. The author does a great job of gradually revealing the true nature and potential of this power as the story progresses. It has an interesting plot with many twists and turns.