Yes, it can be a great concept. Combining Naruto's ninja skills with the Ultra Instinct concept from Dragon Ball can lead to some really exciting and unique storylines. Naruto already has his own chakra - based powers, and adding Ultra Instinct - like abilities would take his power level to a whole new dimension. It could also create interesting interactions with his friends and foes as they react to this new power.
One thing that could happen is that Naruto would become much faster in combat. Ultra Instinct gives the user incredible speed and reflexes. So Naruto could dodge attacks that were previously impossible for him to avoid. For example, he might be able to effortlessly avoid Sasuke's Chidori in a new and intense battle between them.
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.
Goku definitely masters Ultra Instinct in the manga. This transformation gives him a huge boost in power and allows him to take on even stronger opponents.