Disney has announced a new "Disney After Hours BOO BASH" event that takes place on select nights after Magic Kingdom closes (9pm-midnight) on select dates from August 10th through October 31st. Guests can dress up and get their fill of candy throughout Magic Kingdom. Select attractions will open, and there will be themed food and beverages as well as special lighting and music.
Even though it's quite similar, it's unclear at this time if this is replacing Mickey's Not-So-Scary party altogether.
Mickey's Not-So-Scary Halloween parties have been criticised in recent years thanks to higher crowds. However, I have praised After Hours events before, even at their high cost, due to the low crowds. This could be an interesting combination.