PLD and GNB both have a single dot in their rotation. The core the PLD build is a combo of direct damage and magic damage. The core of the GNB is fast paced direct damage. The core of WAR is intervals of burst damage with no pure sustain support in the downtime window. DRK mimics that setup, and its single DOT does not offer overall sustain support. SMN is primarily a DOT DPS class with Direct damage support skills and burst windows built around summoning monster. My suggestion would be a structure along those lines but for tanking. The build would be based around maintaining DOT stacks. Reapplying the DOT refreshes that entire stack. Having a summons that can produce the dots as well and then have its own burst window of consuming the DOTS or giving the player the right to choose when to explode the DOTS would make the shadow more interactive.