So, I qued up for a game today to play for fun. 7/10 people had Dark Harvest. The Dark Harvest meta is even worse than the electrocute one, because literally any champion can just get Dark Harvest and benefit from it. You get Dark Harvest basically for free. It's good on bruisers, mages, certain marksmen. It even helps on tanks. It just outscales every other rune right now. Why hasn't this been nerfed yet? Don't even get me started on some of the REALLY dumb bugs and exploits with it. The first one that comes to mind is that Illaoi can get a Dark Harvest stack from her E's ghost. Yeah, literally. Even when the champion the ghost is from isn't below 50% health, she can still get a stack from getting the GHOST below half health. In that game, even the enemy support had Dark Harvest. He was Blitzcrank, but still. It's just too easy to get stacks, and benefits any champion who would want to use it. It's free burst for doing nothing but hitting people who are below half health.
If I was to propose some kind of change, why not make it so you have to land the killing blow on a champion to get stacks? So the proc still happens at 50% health with the burst damage, but to gain more stacks you actually have to get the kill. This would make it more beneficial to assassins and bruisers, and less accessible to things like poke oriented mages and tanks and supports.