I never played the original so I have no idea how it compares but Kiwami feels more than anything like a big expansion to 0.
Locations, art assets, music, animations and interface elements are all heavily re-used sometimes without any changes at all, sometimes to its detriment (I would find myself thinking that some locations should have changed more in 17 years.)
They have clearly added a lot of reference to 0 into the dialogue, to the point that it feels like 0 really is the new starting point to the story.
But it’s all-round smaller. Shorter (or so I hear, I’m not through it yet), based on one character and main location, and so far with less elaborate side quests and minigames.
It’s all very well done, but sometimes I have to remind myself that this is actually the first story on in the series, because it doesn’t feel that way. Very curious about how 6 next year will feel similar or different.