Are you out of your god damn mind? Did you just propose to add an item that will make mages have a decent chance deal with assassins (especially ad ones)?! SHAME ON YOU.
But in all seriousness, riot will never do it unless they properly address the problem that ap items have. That they are being shared by all mages. Under NO circumstances should a vlad and xerath be able to buy the same items to be viable. One is a mage that sacrifice mobility and tankness for range and
damage, while the other one is a mage that sacrifices range for damage and drain tanking capabilities. They shouldnt share the same items.
And to those that are going to mention " BUT MAGES ONLY CARE FOR BIG NUMBERS (ap specifically) HERP DERP", well riot can avoid this by making INTERESTING PASSIVES. By making us CHOOSE what items we want to build not like the lost chapter shitfest that we currently have (you go seraphs if you are anivia, ryze, cass, signed or ez. You should always never pick glp because its pure garbage UNLESS you really need the extra mana and you cant wait for seraphs.. And you pick ludens when you cant buy seraphs).
Here's an example, if you hit someone that is close to you with one of your spells , you gain (lets say in our case) a stack that increases your armour by y (max stacks being x). The item would also have generally stats that increase mostly the survivability. As such champs that want to deal dmg wont want to buy it most of the times. And in order to avoid (or at least deter) tanks from buying it, they could make it to have a limitation of some sort. Like warmongs. Like, in order for the passive to work, you need to have 150 ap at least or something like that.