I think your concept is maybe a little too niche. I say this to you as a person who spends a lot of time shooting down his own ideas. This one reminds me slightly of one I thought up that I themed around a metronome! I will give you my reasoning for what I think about this item.
First off, kudos to you for not being to **** to understand that the concept is more important than the details. I do like where you were headed with the idea.
Stack duration would be a problem for a few reasons. First of which being global cooldowns and cast times. While most of the AD Casters are instant cast not all of them are. All of them have cast animations some of which they cannot cast during. Given a 0.5 second stack time, it may be impossible for certain champs to utilize.
Infinite stacking. I get that the point was to reward efficient combo casting, but you have to ask yourself one question...Outside of a Katarina landing kills, who can ever stack this passive past 4 stacks? This isn't even considering that the 4th will always be wasted. An AD caster has potentially 4 skills that they can cast, the first of which sees no reduction. Given the cooldowns of skills and the fact that they cannot be reduced below two seconds, the only way that the effect could be perpetual would be to have enough stacks so that all skills are at a 2 second cooldown [including your ult] and casting at the exact moment your stacks would run out.
Efficiency of cooldown reduction. Given that your stacks will only ever reach four and only apply on 3 of your skills [netting you six seconds of overall reduction], wouldn't it be simply more effective most times to simply buy a Brutalizer or another form of CDR? CDR probably matches this alone on your ULT.
It's not that your concept is horrible, it's that the details don't function in practice. I would recommend reconsidering the balance of your item. A stack duration change will help your idea a long way. A long enough duration to be utilized by AD Caster, but not too long as to be good enough for AP casters to use. You could also find another way to incentivize it's use by AD casters further by tying in an action that is more prevalent to them such as auto attacking, or dealing physical damage. You could also consider standard rotations and have a larger bonus per third cast [choose whether or not the focus of this item is to provide amazing reduction for your ult or to reward quick action with your burst combo].
My only real fear for an item like this is counterplay. I can take a general guess that the stack duration regardless will have a smaller window than most hard CCs. Meaning that it's use could be hampered when against a CC heavy team!
Goodluck to you sir and I hope my thoughts help you somehow!
Oh, nevermind what I said about infinite stacking. I think I get what you are doing now and that is fairly powerful reduction. Then I have to ask, doesn't this item favor champs with lower cooldowns to begin with along with resourceless champs?
Also, my bad for not catching your entire vision. You stating that the item stacks infinitely makes it sound like a goal you intended the item to reach! I also believe that this item would have broad implications outside of it's use on AD casters.
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