I don't think reducing the cooldown would help the main issue of sustainability or be healthy for the game. MP regen isn't necessary either, but would certainly help the defensives since TBN consumes MP meaning it's cooldown is actually shorter than it's realistic uptime. The main issue with DRK is sustainability. I'm pretty sure that, because of TBN and its cooldowns, technically DRK reduces damage taken by the largest amount, the issue comes from regenerative abilities and its lack of in long term situations. Every other tank can heal, then when they are done healing, they can heal until their heal is ready. DRK can just heal once off an oGCD every 60 seconds and once every 2 minutes. I think that because of this a better solution is to just throw a small HoT on Oblation or add heals to your Delirium combo and make Dark Mind do something useful. As it stands the main issue with Dark Knight is it just runs out of mits and cannot keep itself up long enough to see them come back.
But also, 2 charges on Dark Arts would be very nice.