My guess is that they are looking for ways to increase night time attendance in the other three parks first. Putting in anything new to MK despite the hub would be counterproductive to spreading the crowds out more evenly.
I would think once RoL opens properly, and the Holiday rush is over they may be better placed to introduce it.
I think making MK even more popular at night now, while there is little reason to be elsewhere would be foolish. In my view a replacement for RoE would make more sense first or if not soon after a Wishes replacement.
Anything new at night in MK is going to be daft if there is nothing elsewhere to compete.