Mages naturally have the ability to deal with assassins since their burst can match an Assassin's at times and CC can be found much more often in a Mages kit then an ADC's to better lock them down, so why you would need an item on top of that I don't get.
Well yeah, that's what I meant. Kit per kit, Mages can and in some cases need to match up to Assassins, so when Assassin itemization biases the fight
, Mages need something in their favor as well
.
Second, Riot has already made an attempt at this sort of itemization in the past with the addition of death's dance in preseason, it however flopped and became a light fighter item instead.
Death's Dance also had a unique form of Life Steal that favored ability casters over hyper carries, along with CDR for utility over raw punching power. It's odd to imagine it as an item to protect ADCs from Assassins when in fact Death's Dance doesn't even support the type of ADCs that most need defense from Assassins.
Last thing, it seems you think I feel an item should be made to completely negate an Assassin when they try to dive an ADC, that is not what I think should be done, I more think an ADC should have something they can opt into if they are struggling; something akin to QSS'ng Zed's ult, but with more of a cost like how Zonyas freezes you in place.
Then build an Armor HP item, or a defense item with a powerful passive like GA. It really wasn't that long ago that common practice for ADCs was to only have a core of 3 damage items, boots, and 2 reactive purchases like Randuins, GA, or Banshees.
And let's be real Zed was the only viable assassin until mid season came around.
Then tailor the thread towards Zed and trying to solve Zed specific problems rather than putting every Assassin's balance in peril with this stupid need for a crutch of an item.