This is an interesting discussion.
Technically-Frankly, I don't think there's a need for a Series III on a current fixed pickup system.
Therefore, I believe a Series III must have physical characteristics enabling, and liberating sounds in ways, not possible/impossible with a Series I or II.
The kill shot for me, is a sliding pickup system. For me, this is the next evolutionary step for a series basses, without the risks of tampering with electronics.
The sound variations, would be unlike nothing ever duplicated from a Series bass; and while keeping the original sounds optional, and sliding the pickups in a physical way, changes things in the most natural and organic way.
Peace and Love,
Hal-