On a large screen, dragging the current window to the snap areas might be a long distance. With the snap preview, it would be powerful to have the snap areas show up centered at your cursor, where you're dragging the window of interest.
The red rectangles in this screenshot are a mockup (imagine my cursor is near the top right of the browser window):
Of course, it would only make sense if the user puts their snap areas close to each other like mine. There are many solutions to this, which I'm sure you can think of too. The, ahem, central idea is to have the snap areas near where you're dragging the window.