I'm incline to think Dark Knight can be "fixed" with one simple addition; Minus Strike.

Essentially the reverse of Spirits Within or Essential Dignity; Lower the Dark Knights HP, the more damage it deals. That has some amazing synergy with other skills (Living Dead and Sole Survivor particularly) and plays perfectly to Dark Knights "strengths".

Potentially very over powered, but in order to achieve that Dark Knight needs to put themselves (and their healer, who'll need to cure bomb/blow cooldowns to keep the Dark Knight alive) in a very bad place, which I think balances it out. I'm actually perfectly fine with Dark Knights lack of utility, my main concern is that its poor survivability/self-healing has zero pay-off. Something like Minus Strike adds a pay-off to how Dark Knight currently is, it transforms the Job from a weak tank with no utility, to a weak tank with no utility that might very well kill you if backed into a corner...

By no means make it as powerful as Bravely Defaults version (because that would deal 14k damage...), but something that scales similar to Essential Dignity would be nice... I certainly wouldn't want to one hit anyone with Minus Strike, I'd just want it to augment my burst and stack with my existing skill set nicely; Ideally I'd want to drop into Walking Dead and have it facilitate enough burst to make a Sole Survivor + Mercy Stroke combo a thing of wonder... Used badly, Minus Strike would be a fart in the wind before you die, used well it could be amazing... The impact that could have on something like Feast would be much more interesting than just straight up giving the Job some utility, IMO...