I was doing some research about spiritual successors and the main thing I've run across is this.

Gameplay, personality and art.

Personality is like the tone, atmosphere, etc.

From my understanding changing over 50% of these elements from the original is what makes it the same as the original or not.

In my opinion a big one would be the plot, if it's story-driven -- having the same plot with just minor differences would probably preclude it from being its own thing.

Yes, that means that games based upon classical folk tales are all pretty much the same in my view, because you know what's going to happen to whom and when.