Will there be dynamic or variable separation minima in future?
There already is, to some extend.

Firstly, there is a big difference between separation in surveillance environments and non-radar environments. In non-radar environments, the separation minima are much larger, because separation is based on position reports provided by the aircraft, which is deemed less accurate than radar data.

In radar environments, the separation needed is generally 5 NM. However, in many terminal areas this is reduced to 3 NM - but only for aircraft going 250 KIAS or less (so, reduced relative speed). Moreover, on final approach, separation is commonly reduced to 2,5 NM - again, since the planes are going slow and pointing in the same direction (the runway), the relative speed is low.

Note that the navigation capabilities of individual aircraft does not matter, since separation is based on ground based equipment (radar), NOT on position information provided by the aircraft.