I don't rightly understand the OP's desire to have Grit and Darkside fall off when MP hits 0; as well as the drain effect... are you mad? That's actually the only change from 4.0 that is even remotely decent for our job, and returning to that sort of pointless resource management; when we now have a gauge to manage, would be beyond excessive. Especially considering you lose the damage increases from Darkside if you bottom out your MP, but don't drop Darkside itself as on 4.0. We can't get any better than that as far that skill is concerned and forcing a Dark Knight to drop their tank stance due to an unrelated resource is just... silly.
My wishlist for the jobs is fairly standard I'd say:
For starters our MP costs are too high. Dark Passenger being 2400 MP is just excessive and using Dark Arts on it feels like a waste as you'll be bleeding yourself dry of 4800 MP. Yes, MP is plentiful but the costs for our MP skills are simply too high and create a feeling of the job flow being disjointed—it's not fun.
Also speaking of DA DP, the blind effect is pointless and should be replaced entirely, the MP cost for DP should also be removed and instead be replaced with a cost of 25 Blood while placing it on a 30s CD and upping the potency; rather than the 50 that all our other gauge actions use.
Unleash should have its potency restored and be given a Dark Arts effect to make it more compelling to use in an AoE situation rather than "Spam Abyssal to Win."
Blood Price's MP gains are garbage and need to be restored to justify the skill's further inclusion in our kit, otherwise the MP effect should be removed or replaced. Yes it is good for Blood but it is practically worthless until you get Enhanced Black Blood is which a problem for early leveling—when it is introduced.
Shadow Wall's recast time is far too long and should be made to have the recast time same as Sentinel.
Dark Arts is vastly underutilized, despite supposedly being what makes us unique and should be used for far more than just the few actions it has.
They should add more gauge related skills, much like WAR, and vary the costs between 25/50/75, rather than just having them all cost 50 blood.
The Blackest Night was supposed to tie everything together, but it doesn't, we need something more, a change somewhere to make play more cohesive and compelling. I'm not about to suggest a change like that, as there are far more qualified players and individuals who could think up a much better solution than I, but the obvious changes they could make are the ones they have ignored and mostly consist of the pointless nerfs we received at Stormblood's release.
Now...if I were to suggest a major change, it would be a Dark Arts effect that spreads The Blackest Night to all party members within 10y of your target as a 5% shield and removing the Death penalty from Living/Walking Dead; as no other invuln will outright kill you if you aren't fully healed... its just dumb.
Our base identity should be High Risk, High Reward yet that should not come at the cost of having a half-baked toolkit that has little to no synergy between our actions and skills. They stripped us of all the skills that made us a Dark Knight and now we're just edgelords flinging around swords that provide very little to a raid group.