Even take something standard and mandatory like mitigation. Mitigation doesn't always have to be fixed %DR. Perhaps excess healing over cap gets converted to temporary HP. Perhaps you have a flat damage shield barrier. Perhaps your %DR ramps up as you take more damage. Perhaps incoming damage gets delivered in a delayed fashion by being converted into a DoT. If you come up with something fun, why not make it a theme across the whole job? Shadow Wall, TBN, Oblation, and even Dark Force have a running visual theme across them featuring magical bubbles with inscriptions on them. That's great, but what if it was a consistent visual signifier of something like 'absorbs flat damage'?
Going in a different direction, the first DRK quest carries a vivid description of a Dark Knight, armor soaked in their own blood as well as the blood of their opponent, transforming itself into a twisted aura. Could you create a tank mitigation theme around that? Absolutely. Just develop it and expand on it in a consistent fashion through the skillset and animations. But when you start throwing in random vampiric effects, random barrier effects, random %DR effects without any real consistency, then there's no coherence to the job.