Beerus. As the God of Destruction, Beerus has incredible power. A story could be written where Beerus decides to test Goku's Ultra Instinct to its limits. Beerus' unpredictable nature and vast power would make him a formidable adversary for Goku.
A new character from an unknown universe. This character could have powers specifically designed to counter Ultra Instinct. For example, they might have the ability to disrupt Goku's mental state, which is crucial for maintaining Ultra Instinct. This would create a really challenging situation for Goku as he tries to figure out how to defeat this new and unexpected foe.
A group of rogue Saiyans from a parallel universe might also be antagonists. They see Goku's Ultra Instinct as a threat to their own plans of universal domination. They have their own unique powers and battle tactics, making them a formidable foe for Goku and his friends.
One idea could be Goku exploring the full extent of Ultra Instinct in a new tournament. He could face opponents from different universes who have developed unique abilities to counter Ultra Instinct. This would lead to intense battles and Goku having to dig deeper into his power.
Vegeta is also very popular. His fans often write fanfiction where he either rivals Goku in achieving Ultra Instinct or develops his own unique form that is just as powerful. His pride and determination make for great storylines in the context of Ultra Instinct.
In some fanfictions, a plot could be Goku trying to teach Ultra Instinct to his friends. Vegeta, in particular, might be a reluctant student at first but then realizes the power's potential. This could lead to a whole new dynamic among the Z - fighters, with them all striving to master this god - like ability.
A fanfiction could explore Gohan's Ultra Instinct in a training scenario. He could be training with Piccolo or Vegeta, and they are both amazed and a bit envious of his ability. Gohan then uses his Ultra Instinct to help them understand new ways of fighting and power - up techniques. This would strengthen their bonds as comrades and also make them more formidable as a team in future battles.
Often, fanfic writers will create a scenario where the training methods from Naruto are applied to Dragon Ball characters to help them reach Ultra Instinct. Maybe a Dragon Ball Saiyan goes through Naruto - style chakra control training which in turn helps in mastering Ultra Instinct. It's all about creative and wild ideas in fanfiction.
Well, with Ultra Instinct, Goku would be able to react extremely quickly to the DC heroes' powers. For instance, when facing Flash, who is super fast, Goku in Ultra Instinct might be able to match his speed or even anticipate his moves better. This could lead to a mutual respect between them as they test each other's limits.
One of the most beloved tropes is the fusion concept. Fanfic writers might create a fusion between a Dragon Ball character and a Naruto character, and this new being has a unique way of achieving or using Ultra Instinct. It combines the best of both worlds, like the Naruto character's strategic thinking and the Dragon Ball character's raw power, all while having an understanding of Ultra Instinct that is different from the original Dragon Ball concept.
Another possibility is a rogue Saiyan from an alternate timeline. This Saiyan could be jealous of Goku's status as the strongest Saiyan in his own timeline. So, they use time travel to come to Goku's timeline and try to defeat him and take over as the dominant Saiyan.
Yes, it is possible. In fanfiction, writers have the creative freedom to reimagine the origin of characters. Goku being born with Ultra Instinct can add a whole new layer to his character development. It could mean that his power growth would be different from the original story, perhaps faster or with different challenges along the way.