Who else would have been interesting as Black?

[SupremeKai25]

But, he does know who Goku is, right? So by researching on Goku, he would learn of Gohan.

He has no logical reason to be interested in Gohan.

Gohan's latest accomplishment was defeating Cell, who is a random android from a backwater planet, from the Gods' perspective.

Goku's latest accomplishments are praise-worthy by the Gods. He unveiled astonishing power in U6 tournament and all Godtube saw it. He also defeated Majin Buu, a threat that killed many Gods of U7. In the Manga, Zamasu praised the one who defeated Majin Buu and couldn't believe that a mortal did it. From the Gods' perspective, defeating Majin Buu was a truly titanic feat that no mortal could accomplish.

Zamasu has reasons to be interested in Goku, Zamasu has reasons to be fascinated by Goku's strength. Zamasu has no reason to be interested in Gohan, because Gohan is not famous amongst the Gods. Meanwhile, Goku was famous for defeating Majin Buu and showing great power at U6 tournament.

Since Zamasu asked Zuno to tell him everything about Goku, he would reasonably know about Gohan too. He certainly knew about Vegeta, because he remarked Vegeta's presence when he pursued Trunks to the Present, despite never meeting him before. However, he has no reason to be interested in Gohan.

[Bloodthroe]

The reason to be interested in Gohan is because he was the Saiyan woth the greatest potential. Greater than Goku.

[SupremeKai25]

The reason to be interested in Gohan is because he was the Saiyan woth the greatest potential. Greater than Goku.

Zamasu doesn't know this and has no reason to care about Gohan's potential.

If he ever heard the word "Gohan", it was only because Zuno mentioned that Goku has sons. And that is headcanon.

This why Goku Black makes the most sense. Goku is MC, Goku is the spotlight, Goku will always be the main player. The other saiyans are unworthy of Zamasu's attention. Gohan? Gohan is a side character, he is irrelevant in the grand scheme of things. Zamasu has no reason to care about body of a side character. The stakes are highest when it's the body of the MC, the protagonist, that gets stolen. Not that of a side character.

[GreatSaiyaman123]

I think Goku Black was the best option for a story.

Sure, Gohan would mean more for Trunks, but Goku means more for the audience. Goku Black is the "Evil Goku" trope brought to canon. We have already seen Evil Vegeta, Evil Piccolo, "Evil" Jiren (Not really evil, but still antagonistic) and it wouldn't work. Evil Gohan is bringing Gohan to the forefront for a role he doesn't fit. It would be really weird for Gohan Black to be fighting everyone while the real Gohan isn't even in the story.

And as far as Zamasu's reasoning goes, Goku was the one using the powers of the gods and addressing the Lord of Everything with little respect, and in the anime Goku came to Zamasu's home and humiliated him. Goku's sins are personal to Zamasu.

[Bloodthroe]

Zamasu doesn't know this and has no reason to care about Gohan's potential.

Well, yeah. You could adjust the story to where Zamasu learns about Gohan, or, adjust his wish to ask for the Saiyan body with the greatest potential. That's what this thread is all about.

[Civic]

I think Goku Black was the best option for a story.

Sure, Gohan would mean more for Trunks, but Goku means more for the audience. Goku Black is the "Evil Goku" trope brought to canon. We have already seen Evil Vegeta, Evil Piccolo, "Evil" Jiren (Not really evil, but still antagonistic) and it wouldn't work. Evil Gohan is bringing Gohan to the forefront for a role he doesn't fit. It would be really weird for Gohan Black to be fighting everyone while the real Gohan isn't even in the story

Having thought about it, Goku was the right choice. The only other character I can think of that might have some interesting themes to explore is Gohan Black, but I see it the flipside of your argument. It's because Gohan isn't evil that would make it cool. Goku has never fought his son in that way, so it would be cool to see Goku's reaction to that. You could still run a similar 'tapping into the body's power' story with Gohan, knowing that he still had heaps of latent power that would be explored in future DBS arcs/movies.

Plus the pain Trunks would have to endure fighting a copy of his master. Hell, even just a copy of the present Gohan. The episode where Trunks spends a day with Gohan and sees his happy life was bittersweet, a Gohan Black would twist the knife even more.

Anyone else has either been done before (Vegeta, Piccolo) or wouldn't really make for an interesting or satisfying evil version.