Zack was almost completely inconsequential to the story. (Especially with how we know there's an infinite number of Zacks now). Literally Venom in Spider-Man 3 type shyt where he was shoehorned in for cheap fan service and easy sales. They could've done without him or at least added the whole Cloud realizing what really happened during those 4 years for added story context. It just made him fighting with Cloud feel gimmicky at the end without the story build up outside of a couple extremely brief scenes.
The ending wasn't as bad as the reviews made it. The "moment" was overly sensationalized but they had a decent send off for Aerith at the end with you not sure if Cloud is tripping because of degradation or Aerith's spirit did something to him during the last fight. Kinda annoying how everyone was letting Cloud slide for tripping so much, and he shouldve been scrutinized a little more.
The Temple of Anicents and Cait Sith levels were super trash but a lot of cool bosses there (ex. Demon Wall(s) and Red Dragon fights were insane). Fighting Vincent's Limit Break form right before was crazy too.
The stuff about the Gi creating the black materia was cool and I hope its explored a little more in the next game. I dont remember this stuff at all from the OG game.
I'm guessing Rocket Town and more on Vincent are gonna be in the next game but it would've been cool to see more Turk interaction with Vincent, when he showed up after the fights with Rude/Reno and Liz/Tseng.
Nah, Zack is there to let us see what's happening in the other timelines now that our main one is all kinds of fukked up and Cloud has awareness of it. Now with no Aerith (or at least not with us) we need a window into how our actions are affecting these realities. As you can see, in Rebirth, the more we push on the more these worlds get destroyed. You can skip it after beating the game because there's no need to tease that information out anymore.
I'm of the belief that the Aerith Cloud sees isn't her at all its Sephiroth once more. Now he knows using her will manipulate Cloud will be WAY more effective than if he did it himself. Cloud thinks the Materia Aerith gave him is the key, when in reality, he never actually gave it to Sephiroth at all. I think instead of Cloud falling into the lifestream in the crater, he's actually going to become an antagonist for a portion of the game and Mideel is going to be destroyed by HIM. THEN we see him come back.
I think after playing through it a couple of times, the ending is actually mad crazy and more than what people are taking it for. I actually called that Zack would help you fight Bizzaro Seph, but I didn't expect it in this game. Which means the final battle in the crater is going to be MUCH longer and MUCH more epic.
I personally think the Temple was actually placed and paced WAY better than in the OG. It kinda didn't make sense to be where it was. Why would the most holy site of the Ancients be so far from their capital. It would be like if the Kabbah in Mecca was in Vancouver instead. So that was a good change imo. I also think making that a MAJOR dungeon before the end was great as well. In the OG it felt like a footnote when it should been REALLY important.
On your other points, there's so much you can expand on. Rocket Town's expansion will be good to see. But Wutai is going to be massive. I wouldn't be shocked if a third of the game was centered around Wutai. I think they'll make Vincent's subplot mandatory so we can see Seph's origins and the WEAPON fights will be totally different. So will the Hojo fight.
If there's any place additional content will come it'll be in that final act where Diamond weapon is attacking Midgar. You could expand that greatly.