This might be an odd take but I would've preferred they DIDN'T reveal what the ancients actually looked like and just kept it a mystery. The hint of Hythlo right before Zodiark's battle showed a bit much but it was arguably not too much, all you could gather was they looked vaguely human. This maintained the mystery of what they looked like while giving a bit of a hint but not too much IMO. And we already saw enough of what their world may have looked like with Emet's recreation of Amaurot. Having this mysterious ancient race that was around and wiped out before the present day people, whom you only get a vague idea how they really appeared is something I find very interesting in fiction. I just don't think it was even necessarily narratively, even if they wanted us to see the past, could have just kept the amaurot-type shades representing them.

It would also keep Emet's original appearance a mystery. I assumed his original self looked very, very different to Solus.

But ah fuck it we'll just show you: turns out they were just big humans. And Emet looked exactly the same he just had different hair and no Garlean third eye. Ok whatever. Sure, that's fine I guess. There's nothing objectively wrong with this it just makes them less interesting with that mystery completley stripped away with the most banal answer possible. I would go as far as to argue that if they were just humans it wasn't even worth revealing. It's just not.....interesting. And Hythlo's appearance before Zodiark's trial already hinted at that enough you didn't really need to go all the way and just reveal it completely.