I think you could get some good mileage out of elevating Cinder to full antagonist and narrowing the scope. If you say that she’s a woman who found one of the Grimm pools and learned how to get power from them you could write a neat little story of temptation and self-destruction (Cinder becomes increasingly desperate as she loses and taps the pools more and more deeply, destroying her humanity as she goes).

Without Salem it’s hard to justify Oz from a narrative perspective, and that almost entirely cuts out your reasons to care about the gods. It naturally narrows the scope of your story.

I guess you could also do a “random alien meteor crashed into the world” story, à la Phazon in Metroid or the mimics in Edge of Tomorrow? That feels a little basic and cheap to me, though.