Here is my 2 cents.

Diabolic Eye effect:
5 upgrades: incrementing value by 4: 4/8/12/16/20
Affects all parameters simultaneously:

Critical hit Rate% UP
Critical hit Damage% UP
Acc+
HP% Decrease
MP% Decrease

I don't normally like to post ideas, but one thing I always thought would be good for DRK is an innate damage mitigation JA; borrowing from other existing abilities.

Obturation
5min recast
5min duration

Effect of ability (one of the 3):
1) Parrying Rate: Flat activation rate boost. Increased TP gain from parried attacks.
2) Retaliation attack: Retaliation for DRK.
3) Counter attack: Counterstance for DRK.

Cost of using ability:
1) Increased MP usage when casting spells.

Stacks with any offensive JA, but like Retaliation, will be inactive if there is a defensive JA active.