There are some great ones. Some authors combine the unique abilities of the Gum Gum Fruit from One Piece into the Naruto universe in really creative ways. For example, they might have a character with the powers of the Gum Gum Fruit training at the Ninja Academy, which leads to all sorts of interesting scenarios and interactions with the original Naruto characters. You can also check out some of the reviews on the fanfiction platforms to find the most popular and well - written ones.
Common themes include the idea of an outsider. Since the Gum Gum Fruit is from a different universe, a character with its power can be seen as an outsider in the Naruto world. This can lead to themes of acceptance or discrimination. Also, adventure is a big theme. Using the Gum Gum Fruit powers to go on quests or explore new areas in the Naruto universe is quite popular in these fanfictions.
In the fanfiction, if Harry eats the Gum - Gum Fruit, it could completely change his personality. He might become more carefree and adventurous, similar to how Luffy is. His relationships with other characters would also shift. For example, his friends might be both amazed and a bit worried about his new powers. And he would probably have to learn how to control his stretching abilities, which could lead to some humorous situations as he accidentally stretches too far or gets stuck in a strange position.
In fanfiction, if Naruto ate the Gum - Gum Fruit, he would likely gain some really interesting abilities. He could stretch his limbs like Luffy from One Piece. This would give him a whole new fighting style. He might be able to stretch his Rasengan - holding hand to surprise his enemies from a long distance. Also, his acrobatic skills would be enhanced as he could use his stretched body parts to swing around like a slingshot.
Another concept might be the creation of new spells or magical items inspired by the Gum Gum Fruit. Maybe a spell that gives an object the ability to stretch like the Gum Gum Fruit. This could be used in various situations, such as in a duel or to reach something in a dangerous place.