Umm. I think it was all great as a standalone game. At this point however, it has some fundamental differences that make it a spin-off (or even a pseudo-sequel?) and not a remake.
I'm mostly here for it I think but I have a few narrative gripes regarding are Sephiroth. FIRSTLY... They've had him go One Winged Angel far, far too early? His character was still developing at this point in the main game he had only just taken Jenova, which applies here too, and yet he's approaching fully developed form. How, and why? I can't think of a narrative reason - the only reason I can think of is "to make a better boss fight" and I'm not sure about that one.
ALSO - and this is a big one for me - a huge story beat of the original is that Cloud at the point of leaving Midgar is supposed to be a bit of a joke compared to Sephiroth, and nowhere near able to go toe to toe with him. Seph could swat him like a fly. The entire journey of the game sees Cloud gain real power to the point that he can go one on one with Sephiroth and win, which Sephiroth did not expect.
Sure he didn't win overall here but they were FAR too closely matched. There was a very good reason that you never actually had this sort of battle against Sephiroth until the end of the whole game. It should have been over in seconds.
Perhaps I can head-cannon it as him pulling his punches? Meh.
There's also no reasonable explanation for Biggs being alive. He was ON the pillar when it came down.
I can see there being big changes going forward but I'm hesitant tbh.