The density and cost and power consumption has been climbing with rackmount gear, honestly I kinda see it hitting the point where the gear you can get your hands on gets too expensive or too power hungry.
I think one of the upcoming ryzen APU's in a jonsbo n3 would be a killer low power setup, memory costs will be painful to get a decent amount into an itx board, you could add 1l PC nodes once you have the NAS already though.