One idea could be to cross UI Goku with a character from another popular anime, like Naruto. UI Goku could end up in the Hidden Leaf Village and have to deal with the different ninja powers and politics there. Maybe he'd have to train some of the young ninjas, which would be really cool as his fighting style is so unique.
First, decide which character or universe you want to cross UI Goku with. This is crucial as it will set the tone for your whole story. Then, think about how Goku gets to this new world or meets the new character. It could be through some kind of interdimensional portal or a magical spell gone wrong.
It's hard to say for sure. Both Goku UI and Jiren are incredibly powerful in the manga. Their strength and skills are on par, so it could go either way depending on various factors.
Trunks is another good option. His time - traveling experience and sword - fighting skills could add a new dimension to the crossover. He and Xeno Goku could work together to prevent disasters in different timelines.
In a Goku Naruto crossover, both characters have their own strengths. Goku's physical strength and energy attacks are off the charts. His ability to instantly teleport and sense ki gives him an advantage in combat. Naruto, though, has a unique power system. His connection to the Nine - Tails and his ability to use different types of chakra make him a force to be reckoned with. If we consider a battle, Goku's power might seem greater on the surface. But Naruto has the advantage of being able to surprise his opponents with his ninjutsu and clones. Also, Naruto has a strong willpower that has allowed him to overcome many tough situations, which could play a role in a fight against Goku.
Goku might teach Naruto about the concept of Super Saiyan forms. Although Naruto doesn't have Saiyan blood, he could learn about the idea of unlocking different power levels through intense emotions and training. For example, Naruto could find a way to apply the idea of Super Saiyan rage - like power - ups when he's in a really tough battle and needs that extra boost.
A Goku crossover with the Pokémon world could be really fun. Goku could try to catch Pokémon using his speed and strength. He might also team up with Ash to battle powerful Pokémon trainers. Or how about a crossover with the Digimon world? Goku could enter the Digital World and help the Digimon in their battles against evil Digimon using his energy attacks.
One common theme is power struggles. Since Black Goku is a very powerful character, when he crosses over, there are often battles to determine who is the strongest in the new combined universe.
Another option is a crossover with a different anime world, like Naruto. GT Goku could end up in the Hidden Leaf Village. The shinobi would be amazed by his power. Goku, being Goku, would be interested in learning their ninja techniques. They could team up against a common enemy that somehow has the ability to use both chakra and ki, creating a unique and exciting battle scenario.