Bit-Patterned Media (BPM) and Heat-Assisted Magnetic Recording (HAMR) are en vogue for upcoming areal density increases. We will be hearing more about them as the alluded to Perpendicular Magentic Recording (PMR) half a decade ago.
As for the RPM of Green drives, it is not variable but rather a variable which WD sets (or fixes) to achieve the target spec for noise/vibration/power consumption and so on, given that constituent parts (especially motor) vary in their spec. The less variability, the more units are going to be same.
Whereas HGST, for instance, actually makes multiple RPM models where the speed can actively step down during idle.