Gumball is sure to be one of the main characters, as it's in his name. His family members like Darwin and Anais are also very likely to be main characters.
One plot idea could be that Gumball loses his braces and has to go on a wild adventure to find them before his next orthodontist appointment. Maybe they got accidentally mixed up with Darwin's fishbowl accessories and are now at the bottom of the ocean.
Since it's a fanfic, it may also bring in elements that the fans have always wanted to see. It might give Gumball some superpowers or put him in situations where he has to make really tough moral choices. The new gumball fanfic could be a great opportunity to see the world of Gumball from a different perspective, with new settings, new conflicts, and new ways for the characters to interact.
Maybe it's one of the school bullies who got hold of a magic book and decided to play a prank on Gumball by casting a love spell. They thought it would be funny to see him act all lovey - dovey, not realizing the chaos it would actually cause.
A more dramatic fanfiction could involve Marshall getting into some trouble with a group of bullies. Gumball, being the loyal friend, comes to his rescue. But in the process, they both learn some valuable lessons about standing up for themselves and the importance of friendship.
The main distinction of porn with a story genre from regular porn is the presence of a coherent narrative. Regular porn often jumps straight to the sexual action, while the story genre tries to add some kind of story framework around it. But in the end, both are adult-oriented and feature explicit sexual content.
Well, it's hard to say for sure. The concept of 'porn-like' is quite subjective. Some might consider certain novels with excessive sexual descriptions as such, but it depends on individual perspectives and cultural standards.