What about a purely defensive item that blocks an amount of damage against Ranged Basic Attacks...

With the "Blocking" power centered/playing with around total armor?

This is just a general idea to help Bruisers/Tanks/Supports against the ADCs, but I was thinking that maybe if there were an item with a passive similar to Doran's Shield (but better) that those roles would be able to do their jobs a little better against the ADCs (which plenty of people have been complaining and pointing out how incredibly strong they've gotten lately.)

The problem with such an item is that while it benefits who its made for it'd also benefit the people who really do not need it - Assassins such as Zed as an example.

So to stop this I was thinking this item could instead interact with the total Armor the of the person using it, with the passive itself being fairly low/moderate but really becoming strong if the player invests in armor.

Assassins and the like would be discouraged from taking the item then as they would have to risk lowering their damage output for more armor (making them weaker against other characters and increasing the possibility of them not being able to kill a target in time), or the item wouldn't be too beneficial for them and allow ADCs to still melt them if given the chance/opportunity.

I do see though that it might be a problem on certain characters (Jarvan, Lee Sin, anyone who can build nearly full tank and still hit like a truck).