I have 2 different Ideas for affliction, one of them is very simple and the other one is more complex.
The first one:
A passive Enemy Debuff called Deteriorating Soul , The Soul of the enemy slowly deteriorates over time grants a Soul Shard Upon reach 50 stacks and Multistrike chance ( or X% Stat, Maybe Crit ) against the target will be increased by 25% for 5 seconds. After the 5 seconds stacking starts again from 0. Every tick of Corruption, Agony, and Unstable affliction will Grant one stack of the debuff. ( partial ticks will not produce stacks ).
This will allow us to have a steady flow of Soul Shards through the entire fight and it will also give us another spec mechanic to monitor. It also fits the theme because the target is slowly dying from our DOTs it must have some other side affects too.
My 2nd Idea:
Increased Corruption: Passive Enemy De buff that has different effects at different stacks, The stacks are applied similarly through dot ticks:
25 Stacks: ( Weakened Soul ) Damage Taken Increased by 10% and Movement Speed Reduced by 15%
50 Stacks: ( Unleashed Corruption ) Enemy's Corruption has reached significant levels where it has started to radiate onto nearby targets, all nearby targets within 10 yards will take damage equal to 30% of UA, Agony and Corruption tick damage. ( this will help affliction AOE )
75 Stacks: ( Inner Conflict ) Enemy's Soul reached the state of madness where it has started to damage itself, a very strong Dot that does x% spell power damage for the next 12 seconds until the Enemy's Soul regains its sanity and its stacks reduced to 0. Inner conflict will give us a very fun and nice window to spam our haunt.
Sorry for the bad names, I didn't have much time to think of something good, I hope you enjoy these Ideas because I would like to have one of these mechanics. I think Chogal had a mechanic where we would get a different debuff every 25 stacks so it isn't impossible to implement this mechanic, for multidot issues I think an ICD can be implemented.