I had this idea when I was driving around last week, it's basically a passive replacement of Harvest Life which allows the Affliction Warlock to continue AoE rotation (SB:SoC, SB:SS spam) while passively channeling Harvest Life.
Glyph of Harvest Souls
While Unstable Affliction, Corruption and Agony are active on 4 or more targets, Harvest Souls activates.
The glyph doesn't really fit with modern glyph design, that would be both a huge dps and healing increase, and since it ties in with talents, would not only make the glyph one of those "mandatory dmg increase glyphs", but also make the talent mandatory on any fight where it could be used.