We are not talking about the DC universe. This is still a Superman movie. We are not working with history, we are working with a clean slate. Here's the thing about MoS, it went out of its way to try and establish itself as being a 'real world' version of Superman, like it or not, any characters that comes here on after have to fit that world - and magic in the form of spells, gods, and demons, doesn't exist in the real world. Something along the lines of dimensional portals or some other theoretical higher dimension could explain Paradise Island and how it remains invisible, how WW gets her powers can be explain through Kryptonian bacteria merging with the ancient population of said invisible island, but 'magic' does not fit in the real world. The thing you've got to remember also is that there isn't any guarantee things will move beyond Batman getting a solo film, WW is only marginally closer to getter her own solo film than what she was a year ago. It's very possible that we don't move far beyond the MoS world.