Something to consider about this, while one of Dark Knight's earliest related mechanics was "Cast From HP" style abilities, in some entries they also possess the ability to drain enemy health. As a few examples, Gafgarion's Night Sword (I think, can't remember the name of the one that drained MP) was added to the Dark Knight class in FFT's remake, Dark Knight in FFX-2 and FFXI can cast Drain, and in FFXI they also gain a unique "Spikes" spell that drains enemy HP to them every time they're hurt by a melee attack.

Just so we're clear on this, what I'm proposing is quite different from what Warrior does. Dark Knight would augment their tanking ability not through self-healing, but rather through draining enemy HP. I suppose they could also gain some HP sacrifice abilities, but I see that being more useful for soloing or off-tanking, or perhaps in fights where its impossible to properly tank the boss anyway, like Giruveganus.

But yeah, that's just my two cents on the matter. If they do that, I could still be happy playing Dark Knight, knowing they still kept the job at least marginally close to its tradition.