Vegeta can also be extremely powerful in these fanfictions. He's constantly striving to be stronger than Goku and might gain some new abilities or techniques for the tournament. Another character could be Jiren. In the original Tournament of Power, he was a very tough opponent, and in fanfiction, he might be even more powerful or have some different backstory that makes him a formidable force.
Goku's ultimate form in the manga is Ultra Instinct. This state grants him enhanced combat abilities and reflexes. It's a breakthrough that showcases his potential and is a key factor in his toughest battles.
Goku's final form in the manga is Ultra Instinct. It's a state where his body and mind work in perfect harmony, granting him near-unbeatable abilities and reflexes.
It could be insanely powerful. Goku already has incredible power levels, and Goku Black is no slouch either. When they fuse, it's like combining two nuclear reactors. Their power could be so great that they could effortlessly destroy planets just by flexing their newfound power. They might be able to reach Super Saiyan forms that are far beyond what we've seen in the official series, with power levels that are basically off the scale.
In a Goku Black crossover Naruto fanfiction, power levels are a bit tricky to compare. Goku Black comes from a universe where power is measured in ki and he has access to super - saiyan forms which boost his already great strength. Naruto has his own unique power system based on chakra. Initially, Goku Black would seem more powerful as his attacks are more destructive on a large scale. But Naruto has an advantage in his ability to adapt and his connection with the Nine - Tails. Also, Naruto's friends and the entire shinobi world could potentially back him up. So, while Goku Black may start as the more powerful one, in the long run, Naruto's growth potential and the support of his world could make him a strong contender for the most powerful character in this crossover.
Their fused form might have Goku's spiky hair, but with some of Goku Black's purple - tinted highlights. The body would be muscular like Goku's, but with a more menacing stance similar to Goku Black. The clothing could be a combination of Goku's orange gi and Goku Black's black under - shirt, maybe with some red accents added in for an extra pop of color.