Well, it's unique because Garlic Jr isn't as well - known as some other Dragon Ball villains. So a fanfic can really delve into his character in ways the original series didn't. It can give him more depth and complexity.
Another potential rival could be a young, prodigious martial artist from a new school of martial arts on Earth. This character could be jealous of Goku Jr's fame and constantly try to outdo him in battles and in the eyes of the public. He might use unorthodox techniques that Goku Jr has to quickly figure out how to counter.
In many Dragon Ball Z fanfics about Garlic Jr.'s death, one common scenario could be that he is finally defeated by Gohan. Gohan, with his increased power and determination, might outwit Garlic Jr. in a battle. Garlic Jr., who is often depicted as a power - hungry villain, could make a fatal mistake during the fight that leads to his downfall. Maybe he underestimates Gohan's new abilities or overextends himself in an attack.
Well, it could be about Goku Jr learning some of Goku's most powerful moves. Maybe Goku teaches him the Kamehameha. They might also go on an adventure together, like fighting a new and powerful enemy that neither could face alone.
A fanfic might also have a plot where Garlic Jr. makes a deal with a more powerful villain from another dimension. However, the deal backfires on him. The other villain double - crosses him during a battle with the Z - Fighters. This leaves Garlic Jr. vulnerable, and he is quickly dispatched by Goku or one of the other heroes.
One interesting 'Dragon Ball Garlic Jr Fanfic' could be the ones that explore his backstory more deeply. Maybe a fanfic where we see how he became the villain he is in the Dragon Ball universe. It could show his early life on his home planet and what led him to have such a vendetta against Goku and the others.
To develop Goku Jr's character further, he could be given more complex emotions. For example, showing his fear in the face of truly overwhelming enemies, rather than always being fearless like Goku. This would make him more relatable. Also, exploring his relationships in more depth, like his friendship with Pan. Maybe they have a falling out over different views on how to protect their world, which forces Goku Jr. to grow and understand different perspectives.
Maybe an old enemy of Goku's who has somehow managed to return. This enemy has heard about Goku Jr and figures that by attacking him, he can get revenge on Goku. This could be someone like Frieza who has found a new way to be resurrected and is more powerful than ever.