One possible plot could be that there is a rift in the multiverse that brings Goku and Luffy together in a strange new world. They initially misunderstand each other and start to fight. As the fight progresses, they both realize that there is a greater threat at hand that is causing the rift. So, they team up, with Goku teaching Luffy some of his combat techniques and Luffy showing Goku the value of friendship and loyalty in a different way.
Goku in Ultra Instinct mode is extremely powerful. He has superhuman speed, strength, and can dodge attacks effortlessly. However, Luffy also has his own set of amazing abilities like his Gear Fourth form which gives him enhanced speed and power. But I think Goku would win because his combat skills and power level are just on a different scale compared to Luffy.
In 'luffy vs kaido fanfiction', Luffy's growth and new abilities are often explored. He might develop new forms of his Gear powers to face Kaido. Kaido, being extremely powerful, forces Luffy to push his limits. Fanfictions may also focus on the intense battle scenes, with detailed descriptions of their attacks and counter - attacks.
Perhaps there is a group of characters. There could be a mentor figure who has a high power level and helps the protagonist, who starts with a much lower power level. And there might be antagonists with their own distinct power levels that pose a threat to the main characters.
Well, it could have various unique features. Maybe it has a complex power system where characters have different power levels that drive the plot. For example, the power levels might determine the social hierarchy in the story world.
In Naruto fanfiction, power levels can vary greatly. Some fanfictions might boost the power of a character like Naruto himself, making his already strong Rasengan even more powerful, perhaps with new elements added. Others might focus on secondary characters and give them powers on par with the main characters. For example, a fanfiction could make Rock Lee able to use some form of ninjutsu on top of his taijutsu skills, which would significantly increase his power level in that fictional story.
Well, you could start by looking at the characters' abilities, skills, and the scope of their influence. Also, consider their achievements and how they fare against opponents.
Goku in the manga doesn't hit a power level of 8000. The power escalation in the story doesn't go that high for him. There are various factors and story arcs that determine his power growth, but 8000 isn't one of them.