Currently the single target state of affliction is a joke, its just a struggle and low reward because of the abysmal Unstable affliction.
It does not do enough damage to warrant going for mastery and its peak talents is laughable.
Make unstable affliction into a constant debuff. Whenevery you gain a second soul shard (always keep one up for burn and/or SoC to spread corruption) It eats it and deals alot of damage to the target with the Unstable corruption.
Would fit as flavour because its unstable, if you dispell it you can still get hurt and it rewards spreading agony and using soul harvest to do more damage on one guy.
The apex talents could be “10% to procc soul shards twice to deal damage” and “Gain 5% haste whenever it deals damage, stacks twice lasts for 10 seconds.
Reward people playing the game instead of making it a struggle to use your abilities. Then you can balance the single target by tweaking the UA’s proc damage and the class would flow better.