What I think they can do to bring back DA and rework DRK:

Remove TBN's MP cost. Shattering TBN gives you a regen, an unshattered TBN heals you for whatever HP was left in the shield.
Bring back DA and give it the 3K MP cost. It buffs the next GCD by a percentage (10% maybe?).
Exchange Flood and Abyssal Drain so AD has the MP cost and Flood is linked to Carve and Spit.
AD gets a bigger cure potency, but gives no darkside.
Make Bloodspiller and Quietus cost Darkside duration to use (15s worth).
Delirium allows the use of Bloodspiller and Quietus without Darkside cost. It also gives a buff allowing use of Shadowbringer for the next 30s.
Living Shadow copies the next 3-5 GCDs the DRK uses at some % potency (so it's a gain, but not overwhelmingly so).

What this does is make Darkside duration another resource we're juggling. TBN is no longer attached to our damage but is still worth breaking. Our MP pool becomes used in one of 3 offensive ways, Edge for Darkside, DA for the buffed attacks, and AD for AoE damage with a heal. This change makes MP an actual resource we have to think about using instead of spamming Edge/DA on cooldown. This also links Shadowbringer to our kit instead of it being unlinked to anything, and improves the feel of Living Shadow.