You will never balance Dark Harvest

At the very least not in its current form.

The power fantasy at the core of Dark Harvest is that of Nasus. Get enough stacks, one shot enemy champs and chunk towers. That statement on its own should set off klaxons all over the place, but we haven't even begun to scratch the surface of the issues.

Dark Harvest promotes snowballing. No other rune with the exception of maybe Grasp of the Undying gives such immediate benefits that it does. This becomes most apparent at team fights, where a proper string of souls on the ground can guarantee you proc your rune 3-5 times, OR MORE. Remember your own allies give souls as well. That to me is ridiculous. An electrocute that can proc multiple times within the span of 8 seconds or less.

Dark Harvest PREVENTS player interaction. An active Dark Harvest proc means that for the duration of that buff you are LETHAL. In lane, you win trades with ease. Guess that means the opposing player has to wait and play defensively until the buff for Dark Harvest falls off. Except since the champion's own allied minions can give a stack as well, it means it potentially has twice the actual uptime. And at 150, kiss it all goodbye. You can no longer safely approach that champion without taking the proc on account of it being up near permanently. Team fights are your only option now, and pray to god no one dies and drops ANOTHER soul after taking the proc to the face. I want to reiterate that the only requirements for Dark Harvest is an AA. A single one. It is instantaneous and in certain cases like Shaco, Twitch and Akali, you are dead before the animation is even finished for you.

Which brings me to my next issue. The limitations and restrictions that apply to Nasus DO NOT apply to other champions. Other champions have mobility, stealth, hard CC, RANGE. Dark Harvest can be abused in SO MANY WAYS. I know I mentioned invisibility already but I want you to try Dark Harvest with a Rapid Fire cannon on a Kog'Maw some time. Rengar (I could give him his own paragraph.) just needs a bush and he can start his own killing spree. Hecarim running at the speed of light doesn't even need to directly land the attack. He just needs to be able to start the leaping ANIMATION of his attack and it will connect without fail, even if you flash, or if he's CC'd before he reaches you with the dash (This case also applies to Predator mind you.)

I keep comparing the rune to Nasus, but honestly it's no comparison. I mean imagine if Nasus had bonus AD/AP ratios on his Q. Cause Dark Harvest does.
I have seen a Veigar grab Dark Harvest. Let me tell you, when you're running away from Veigar's AA because it will deal more damage than his actual Q, SOMETHING has gone horribly wrong. I know Jhin's been busy with Hail of Blades of late, but I predict that his most popular rune will be switching over to Dark Harvest shortly. The fact that it infinitely stacks AND has Ratios means that even if a player is the worst soul stacker on the US server, he will still chunk you hard late game.

It's getting late for me, and I really could go on so I'm gonna summarize this last bit.

Let's say you fix it so that it now has a Cooldown when procc'd. Lets say you reduce/remove the AD/AP ratios. Let's say you limit that soul essence that drops AGAIN. Let's say we fix it so that ranged damage gets reduced. Let's say we nerf ALL OF THAT.

You can't fix the Power Fantasy of Dark Harvest. It's to one-shot people. If it can't burst someone down, it's failed its purpose.