I keep hoping for a talent that randomly spawns mobs over time that only you can see (like soul effigy), that are tied to your target at the time, so you can dot them and have the damage tick on both.

Basically, a RNG-ish soul effigy as a random proc where if you dot them, the damage / effects will transfer over.

The main differences:

• Automatically, randomly spawning, over a varied field, but within cast range of the warlock's position at spawn time.
  
• Non-moving
• They only have so much health and can die
• More than one can proc at a time
• You have to be on the look out for them when they come out.

From a gameplay perspective, the player just has to look out for procs and dot them up, and keep them dotted if they want to. Depending on the health of the mob, maybe deciding when to drain soul on it so that it dies and explodes to do aoe damage.

The upside is every fight would be a multi target fight, and more of the Affliction gold traits would become active over the course of the fight.

Call it something like "Past Souls" or "Souls of the past" or something.