It’s hard to explain without spoiling; it’s more of a backstory film.
However if you do want spoilers, here ya go:
The Dirty Lady is actually a good ghost who helps Elise, contrary to how the maze made her seem as a villain. She is also in the movie more than the Key Demon is. The Duck Masked Boy isn’t in the film at all, except for one scene where it looks like he’s running into Elise’s closet. The scene in the trailer where all the ghosts come out of the prison cells and say “This way” isn’t in the movie either, as is the scene where the Key Demon is behind a window. The only scenes in the film that were the same in the maze were the scene where Elise’s mom is hung by wires and the scene where Elise was in a prison cell. Besides that none of the other scenes are carried over. Not even the mom who stabs the baby; I really want to see the film that could’ve been because the movie that came out is still very good, but not really a horror movie.
Edit: I hope Murdy is smart enough to recognize nobody wants Freddy back for a 3rd year in a row. Especially when there are properties like The Conjuring, IT, Trick r Treat, and AHS Asylum that haven’t even been done yet