Bring back the MP/Blood feedback.

Quietus -> Restores some amount of MP per enemy hit
Flood of Darkness -> Gains some amount of Blood per enemy hit

Turn Darkside into a Pseudo Resource

Darkside as a Duration Buff, where the duration is taxed when additional effects are added. Base duration of 1 / 1.

Salted Earth turned into a toggle-able AoE Damage aura. Increases Darkside's rate of depletion.
One or two new abilities focus around this concept.

Maybe: Situational deterrent for hitting 0 Darkside. AoE explosion + X duration of Pacify, as example.

New Charge Sub-type

For Actions that can be used even at 0 charge but gain a specific benefit when a charge is available.

Abyssal Drain: Gains a charge every X seconds. (30, 45, 60, whatever). When a charge is available, Abyssal Drain's MP cost is reduced to 0.
Abyssal Drain is now a Darkside skill.

Active Defense and Retaliation

Dark Mind evolves into Dark Missionary.

Dark Missionary's "button economy" relegated to a GCD cooldown, reducing incoming damage for a short duration and delivering a PBAOE weaponskill counter attack, ramping up as more hits are taken during this duration.