I mean think about it. At lower MPs this would practically go unnoticed but at higher MP levels where the fights are long and deaths occur less frequently, Grave Injustice looses all value. The only deaths that happen in long fights, is death of you, your pets or teammates. So if it could proc on these deaths, it would be worth something.
Should Grave Injustice proc on all deaths?
All deaths including that of Witch Doctor's own pets and nearby players (and their pets). By pets I mean not just Zombie Dogs and Gargantuan, but also Fetishes and minor pets like Creepers (summoned from Zombie Wall), Lob Blobs (Acid Cloud), Spider Queen. I think that's about it.
The cool-down would have to be triggered on the end of the lifecycle, not on recasting. So Spider queen dies after 15s, at the end of 15s, -1 to all cooldowns. If the player repositions her, then it's not until another 15s that the cooldowns are reduced. There are three Lob Blobs that die after 5 seconds. Three Creepers, also die after 5 I think. Fetishes die after 60 seconds, or a few seconds after when they are summoned, at higher MP. Dogs die pretty often. Gargantuan rarely.
This wouldn't be much, not like CM, but it would give some value to Grave Injustice in higher MP.