Interesting idea. However, a quick search reveals that the only spell with which this is possible seems to be Summon Zombie Dogs, and since you have a limit of 3 anyways it doesn't really mean anything beyond "instantly refreshing any of your dogs" (and Mana will be your limiting factor).
Or is there another spell you were thinking of? Couldn't find enough possible bonuses on legendaries to reduce anything below zero except for Zombie Dogs... Teleport for wizards for example has -4 on Oculus only.
I believe it does, yes. There was a WD Sacrifice build here in the forum that reduced Zombie Dogs' CD to 0 seconds, but I couldn't find the thread. I believe there was also a Pile On build that reduced Zombie Wall's CD to 0.
i will go and become a instant type spell, and if i remember right sacrifice is "off the global cooldown".
but the dogs will still apply to the attackspeed limit, might also be a summon animation limit also.
I had a friend who ran the 0-second Zombie Dogs build for his WD. I'm going to imagine it is instant cast because on my screen, I will just see nonstop explosions from Sacrifice. This can only happen if he spams the summon Dogs ability along with the Sacrifice skill back and forth. Not 100% sure if attack speed is factored in here, but I won't be surprised if it is.
It's like the same with wizards summoning a Hydra. It has no cooldown, and each recast plays the spell animation and refreshes the existing Hydra.