If you don't remember blackout was a BC talent for shadow priest that rng stuned people when you did damage. It was 10% chance to proc a 3 second stun. Now clearly that would be far to high now as haste and mastery give us more ticks. However if the talent was a 5-10% chance to proc a 2-3 second stun with a 15-20s ICD it might be balanced. The ICD would most likey be nessarcy to prevent too much haste/mastery scaling along with rng chain stuns. The talent (which would probably replace mind control) would compete with psyfiend. This would give priest more options than simply psyfiend every arena match. Keep in mind the stun would dr with any other stun, so teams like god comp prob would not run it as they risk dr on deep freeze. Given the right tuning on ICD proc chance and stun time it could be a balanced talent and a nice alternitive to psyfiend.
There would also have to be a healer version of the talent. TBH I have not idea what it would be, but prehaps it could be a 5% chance to reset the cd of cleanse after a dricet heal. 20s ICD.