I think one of the easiest ways to resolve the issues with Shadow is, don’t compromise. Give everyone exactly what they want.
Give Priests a 4th spec: Shadow & Void as two separate specs.
Void is Legion Shadow.
Shadow is MoP Shadow.
This shouldn’t be very hard, you slap on a 4th spec, you restore two past designs, you do some numbers tuning, and then everyone is happy.
If you like Void’s Clarity of Power gameplay spamming your two buttons, it’s still here!
If you want to be a DoTty vampire again, it’s back!
They did it for Feral when Bearcat was too thematically complicated: so there is precedent. Splitting Feral into Feral & Guardian has been a resounding success.