Do you know what would be even more powerful? An hypothetical "UI" Fused Zamasu:
Fused Zamasu had all of Black's strengths: Incredible power, excellent swordsmanship, unlimited potential and broken Zenkais. Just think about that time when Fused Zamasu damaged himself with the divine Light of Justice, and got a huge power boost that put him very close to SSB Vegito's level.
But Fused Zamasu was also immortal. This means that his regeneration skills were nigh-perfect. Just consider the fact that a fully-charged Final Kamehameha from SSB Vegito did absolutely nothing to Fused Zamasu, such was the power of his immortality, such was the stamina that his divine everlasting body had. And he would never get tired because of his endless stamina.
Only an attack powerful enough to completely shatter the balance between his mortal body and immortal soul managed to defeat him. An attack which resulted in the destruction of his body. But since killing is forbidden in the Tournament, such an attack would be nigh-impossible to perform because everyone would be scared of killing him.
Another advantage that Fused Zamasu had, and that Black lacked, where AoE attacks. With 80 fighters lumped into the same small arena, Fused Zamasu would have truly shined, because he possessed several AoE techniques such as the Blades of Judgement, the Lightning of Absolution, and the Holy Wrath, that damaged huge areas with massive explosions. Even without trying, Fused Zamasu could have probably eliminated the vast majority of fighters in 5 seconds. Or maybe 4 seconds? I don't know, how much does it take to fire Holy Wrath?
Perhaps UI Fused Zamasu would lose to UI Vegito/Gogeta or UI Kefla. But his great advantage, lies in the time limit. Vegito lasted about 5 minutes just by being in Super Saiyan Blue, how long would his fusion last in Ultra Instinct? 2 minutes? But Fused Zamasu wouldn't have this problem, since he was the fusion between two fully-fledged Supreme Kais, and so he, as a God, would never defuse. He was truly eternal.
But even without Ultra Instinct and his power boost which he got from damaging himself with the divine Light of Justice, Fused Zamasu was so powerful that SSB Goku literally had to BREAK HIS ARMS to push back the mastodontic Holy Wrath which had the power to destroy an entire planet.
If this had happened. everyone in that Tournament would have been fucked:
Just imagine how OP and broken he would be with the ability to use his scythe to create an infinite number of UI Goku Black clones
You know, I've been thinking about this, and I came to the conclusion that that was just a single use only ability. Why? Because Fused Zamasu never used it, even though he retained all of Black's techniques. There must have been a reason why he never used that ability, even though he would have won if he had, because it was just so broken.
Just imagine how OP and broken he would be with the ability to use his scythe to create an infinite number of UI Goku Black clones plus his broken zenkai boosts too! Black and Team U10 would've absolutely dominated the TOP with almost no difficulty even with strong fighters like UI Goku, SSBE Vegeta, Jiren, GOD Toppo, Kefla, Anilaza, etc in the arena as well. I'd say U10's victory would've been all but guaranteed. 
