Zamasu, the then Supreme Kai apprentice of Universe 10 is known to have achieved a lot of power through stealing Goku’s body. He then used Goku’s power to achieve his noble purpose: to eradicate every ningen.
I can’t understand why this guy wanted to eradicate ningens when every sentence he said had “NINGEN” on it. But anyway, the only time Vegeta and Goku were able to fight equally against Fused Zamasu is when they fused into Vegito Blue. Fused Zamasu was really OP because of his immortality.
But what would’ve happened if Zamasu met Jiren first instead of Goku? Will it be more problematic if Zamasu stole Jiren’s body instead? He barely saw Goku’s real strength, and Goku has been holding back during their fight. What if he witnessed Jiren’s might in the first place?
Jiren, even if he’s holding back, is way more impressive than Super Saiyan 2 Goku. Zamasu would’ve done everything to have Jiren’s power.
The first notable thing that we would see if this happened is…Fused Zamasu will be a Jiren with a white mohawk and green skin. (Hey don’t hate me, that’s the first thing I thought and that’s not that bad).
But on with the serious talk, after Toppo and company discovered that their Supreme Kai and GoD was killed alongside Jiren (in Zamasu’s body), they will go after…Jiren Black. He will be likely killed by Jiren Black because Jiren is more powerful than him.
Jiren Black will then eradicate humans on his timeline and proceed to work with future Zamasu. Future Trunks will then go back in the past to fetch some recruitments.
Goku and Vegeta will meet Jiren Black and his overwhelming power for the first time in Future Trunk’s timeline. They will be beaten and tossed and will go back to their current timeline like what happened in the anime.
And just like what happened in the anime, they will go back and get beaten again. Only this time, instead of turning into Vegito Blue for their last resort, Jiren Black’s attack on Vegeta when he shielded Future Trunks did not only injured him but killed him.
Vegeta’s death, Goku’s desperation and exhaustion, and Jiren Black’s overwhelming power, Goku will awaken Ultra Instinct for the first time. He might or might not be able to master it on Future Trunks’ timeline, but it would be a lot more different for the Tournament of Power.
After knowing Jiren’s skillset and Goku’s mastery of Ultra Instinct, we might see Jiren and Goku’s battle in a more exciting light. I’ll stop right here because this post will be a novel if I elaborate any further. But feel free to drop comments about this theory.