Well, it could be about an OC who uses the powers of the Goro Goro no Mi in a world full of pirates. Maybe they use the lightning - based powers to fight against other powerful foes, like the Marines or other pirates. They might also use it to explore uncharted islands that are filled with dangerous creatures and hidden treasures.
Naruto himself is often a popular character in such fanfiction. Writers might explore how he would adapt to having the Goro Goro no Mi power, or how he would interact with someone who has it.
One interesting concept could be how the Goro Goro no Mi power is used in unique battle scenarios. For example, the user of this Devil Fruit power might combine it with Naruto's signature jutsu in creative ways to defeat powerful enemies.
It could be a new character who has a deep understanding of the Goro Goro no Mi's powers and wants to use Luffy's new - found abilities for their own evil purposes. This character might be a scientist who has studied Devil Fruits extensively and sees Luffy as a prime specimen to experiment on.
One of the best novel intros is the opening of 'Pride and Prejudice' by Jane Austen. 'It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife.' It immediately sets the tone, gives a sense of the society's values and also intrigues the reader about what will follow regarding marriage and relationships in the story.
It's a bit complicated. Sometimes certain elements of the MKX comics are incorporated into the overall canon, while others might be more standalone or supplementary. It really varies based on the larger narrative framework and how it all fits together.
It depends on how you measure it. If you count the main plotline, it's a decent length. But if you include all the side quests and character development, it can feel quite extensive.