Would probably be labeled as "too complicated" but I wouldn't mind DRK being a tank that possessed self destructive skills for a sizeable amount of burst damage paired with sustain/recovery based around stealing HP from enemies. FFXI probably hit this niche the best as DRK in that game had access to a majority of the plate tank armor along with a VIT stat and HP pool comparable to PLD and even had a skill with a built in emnity boost in Last Resort which spiked enmity like a small provoke while boosting attack and lowering defense. The kit also had Souleater which converted your own HP into raw additional damage and it's trumpcard ability with a 2hr cooldown Bloodweapon which converted your damage into raw absorbed HP (unless you were stupid enough to try using it on undead cause that game actually had normal JRPG mechanics). It also had access to magic though the only real notable spells were specifically classified as dark magic which included Bio spells which in FFXI was a DoT that weakened the enemy's offense and Drain, Drain II, Aspir, and a set of Absorb spells that would temporarily steal stats from the target like str, vit, dex, etc...

At the same time I feel like a tank with self destructive spike damage skills would probably have alot of healers complaining because they have to pay more attention. I know insta-KO'd myself in FFXI using a 4 hit scythe weaponskill with souleater active while wearing gear items that boost it's effect paired with gear that granted passives for double or triple attack and having one of those proc on every hit of the Weaponskill... >.>; I recall making a macro for souleater that said something along the lines of "Wheee~ Look at my HP fly away!" XD