Solely Goku Black and nothing else from the anime/manga except... power scaling? (Is that the right word?) Time to give
The Retro Kakarotto a shot at this!
But Anime or Manga though? That's the thing. Also ain't gonna cover Merged Zamasu, haven't seen Manga's Zamasu die yet and it's just normal Zamasu at that point but more arrogant and or insane.
Who is Goku Black?
Since the anime was introduced before the manga, it built suspense as to who Goku Black was. (Boy was that my summer wasted trying to wrap my head around that one.) We were certain it was a form of Goku corrupted, we didn't know how. The moment we saw him with the time ring, we jumped to the conclusion of an alternate timeline (but a more comprehensible version). We also thought he was created directly from the Super Dragon Balls. While that was a good idea (and honestly one better than evil Kaioshin, or hell just "summon a powerful Goku" or something) when it was revealed it was just a Kaioshin with deep hatred for Mortals (see Motives below), it just lost some of its luster. It wasn't THE Goku that ended up killing Future Bulma, it wasn't THE Goku that "betrayed" the world, it wasn't THE Goku that Trunks sought help from those years ago
it wasn't THE Goku everyone had to wait for. It was just some poser who had to use a more advance version of body swap. Maybe the Dragon allowed the body and mind to sync faster than Ginyu's technique but the latter's method was already introduced and unique to the Ginyu Force team. Why drag a Kaioshin, a holy being to be reduced to using an ancient relic instead of asking Ginyu how to do it. (Why didn't he? That'd be interesting. Oh no, it's from a mortal, fuck them, I'll use the Godly version.)
In the Manga, we already know who it was so it wasn't as suspenseful as it should've been. Maybe if it was like the old days both were released simultaneously, it would've had a better impact to the audience "Oh my God, Goku Black looks so cool, who is he, I can't find anything about him in the show or the manga!!!" But it's more of a "oh yeah, it's him that's attacking Trunks... meh, what's next?"
Was Future Zamasu, not bodyswapped one, needed at all?
The only good he was there for in the anime was to act as a foil to Goku Black but other than that, he just helped GokuB get the defense that punching "really really hard" won't cut. "Nearly immortal" shit.
In the Manga, iirc, Future Zamasu was actually standing up to Goku Black when "their" (or was it "his?") plan was gonna fuck shit up because it was about to go downhill. But other than that, we need to see the defusion to know how it's gonna go down.
Motive
A guy who hated Mortals because they were committing so many sins, the God who was supposed to make a Utopia ended up committing the same acts of crime. Literally, he went back, AND forward in time to change how things go down in the respective timeline... but multiverse theory saved us a bit. That was an interesting concept, and a reason for that, at least imo, was how I love to hate characters that act hypocritically and how they couldn't see the faults of their own actions. Whether I redeem them afterwards is a different story, depending if the villain wasn't the actual big bad and he realizes his boss' motives was wrong the whole time.
But to explain that he wanted the Body of the mortal that triggered him was a bit sloppy (but we know it's for an evil Goku fanservice) because if he has the balls, why didn't he just erase mortals entirely? In the Anime, however sloppy it was to say, Goku randomly fought Zamasu, at least it was reasonable. In the manga, I can dislike if the button existed, he literally got mad at a GodTube video how a guy was doing so well in a tournament because he was stronger than himself [Zamasu]. That's like getting pissed at a Smash Bros Tournament that you don't even participate in and saying that guy is a douche when you know full well they fought within the tournament rules, it's not their fault they had quick processors in their brains.
Also his last statement how "it's all Trunks' fault" should've made Trunks the target of hate, not Goku... unless Zamasu intertwines the explanation as "I stole the body of the man you sought help from Trunks, look where his power took him,, mwah haha!!!!" but alas, that is lost to time... (Totally worth it
)
Lastly, for now, might add more on Fused Zamasu and Future Trunks specifically, both SS Rage and Healing powers.
Setting of the battle(s)
The Future? Oh my God the world is gonna di- wait didn't the Androids already do that?
It's not original, and it cheapens Black's onslaught of murdering Ningen because the population was already decimated. Maybe if he was targeting Trunks Specifically and how he followed him to the present, made the "final battle" over there (or over "then?") that might have brought tension. But nope, "let's go attack the Future that's already fucked up. Let's not give a shit about reviving the timeline completely to match ours so that our Future Versions wouldn't have died as they did. Let's just fight in this dead wasteland. Oh sorry Trunks, did you say something? Something about 'Too soon Goku?' i couldn't hear you, I wanna fight me so much right now!" Also Maybe go to the root of all timelines and say the Timering's ability can override all these timelines if actions were to change here, that'd bring more tension. Have Zamasu go back to after Zeno's creation to U7 and act like the God instead to all the mortals and fuck with them however he pleases... the insult, not the action. That'd be a true battle to create tension.
Last one, sorry for lyin. Lol. (Also couldn't tell what this should be called.)
Wrong time at the wrong place. It's even worse in the manga how both Zamasus make nods to the Omni-King possibly might see their actions. The anime made Omni-King "too busy" or too not caring to notice this but the statement in the manga makes Zamasu look like a dumbass for carrying out this plan regardless of the fact that the literal higher being is there and can erase him anytime he wants. (Although Gowasu's attempt to correct him and return him to normal while apologizing to the Omni-King is a point in Gowasu's favor. Nice try man, I really do pity you when you got stabbed... that sounds weird.)
tl;dr for this paragraph
Zamasu in the anime looked like a person who didn't know there were drawbacks the 0NingenPlan
Zamasu in the manga looks like a dumbass for acting his plan despite knowing that Zeno could've stopped him