>>718935089
Well the most obvious change from VsXIII to XV was the removal of all the FNC mythos, the Lucis family was originally meant to be devoted to Etro which is why in the final game all the main characters have inexplicable skulls everywhere on their designs.
There's a bunch of cut characters, namely Stella who became Lunafreya, and most of the antagonists, while technically present in XV, barely do anything and it's safe to say they'd have had a much bigger role in VsXIII.