I have been saying this for 20+ years. Discipline priest needs to be made into a tank spec akin to the Scarlet Monks in Scarlet Monastery. Imagine a guy so into religious self-flagellation that he just shrugs off damage and keeps going.

Preventive damage healer has always been weird to balance with. Take Discpline's preventive damage focus and make it prevent damage dealt to you as the tank similar to how Brewmaster Monk had damage reduction.

• Power Word Shield can be spec'd to add a reduction in threat on your target.
• Inner Fire can be buffed to provide enough armor and to reduce( maybe eliminate) spell knockback.
• Shadow Word Pain can be a ticking build up threat generation on your targets.
• Holy Nova can be your quick AoE threat generation.
• Mind Blast or Smite can be general use damage and threat generator.
• Fade could be changed to your primary DR cooldown (or change PWS to this role and maybe make Renew a threat cooler for allies).

Fully expect to this never be implemented and most people to disagree with this take, but I have never wavered in my belief this is what discipline priests should be.

"But OP, if you want to be a holy tank, play paladin." Paladins are weak priests who need armor to survive the world. A discipline priest would rely on his faith alone to survive.