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 possible plot is a crossover with the Naruto universe. Goku in Ultra Instinct mode could be mistaken for some sort of super - powered ninja. He might end up training with the Hokage or getting involved in the Akatsuki's plans. Since his Ultra Instinct gives him such incredible reflexes and power, he could easily hold his own against the strongest ninjas like Naruto in his Six Paths Sage Mode or Sasuke with his Rinnegan. It would be a clash of two very different power systems.
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.
One idea could be exploring how Goku further masters Ultra Instinct in ways that are different from the canon. For example, he might have to face an enemy that can nullify his normal Ultra Instinct attacks, so he has to find new ways to use it, like combining it with Kaioken.
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.
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.
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.
One possible plot development could be Goku struggling to control the Ultra Instinct state for an extended period. This might lead to him accidentally causing damage to his surroundings or even to his friends while in this super - powered state. Another could be that enemies take advantage of his 'stuck' situation, seeing it as a vulnerability to attack him when he can't fully regulate his power.
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.
You could write a fanfiction where Goku teaches others about Ultra Instinct. Maybe he tries to pass on this power to Gohan or Trunks. This would involve Goku having to understand the power more deeply himself in order to teach it, and it would also be a great way to develop the other characters.