The tank item that needs to exist (in regards to balance between incomming damage types)

There needs to be an item available that accomplishes the following:

+25 flat Armor, +25 flat Magic Resist +300 HP Unique Passive: Balanced Defense - Gain Armor equal to 15% of your purchased Magic Resist. Gain Magic Resist equal to 15% of your purchased Armor.

Keep in mind these numbers are completely irrelevant and should certainly be changed for the sake of Balance, I just gave specific values to paint a clearer picture of what I mean.

Generally speaking, the greater the mix of damage types from the enemy team, the less effective tanks wind up being later on in the game. Considering they have to split their gold into both MR and Armor, as well as HP, they tend to fall off every time they buy an item that grants a resist that doesn't apply to the opposite damage type enemy carry. Meanwhile the damage dealers have a very simple build path; More Damage. It's really simple for them, regardless of if they are AP or AD, and they scale at a steady rate regardless of comps.

Basically, the item is intended to assist a tank late game, when they have stacked one mitigation type, and then they find themselves -needing- a little bit of the other resist as well. The item is cost inefficient early on, as the unique passive will do close to nothing, however when a tank has +250 armor from items, they can get +37.5 MR from the passive (+the flat value) to help against those Corki rockets and Support Morg damage, they can get it cost effectively while still being able to gain even more armor / HP for that Riven Top, Lee jungle, and Zed mid, etc etc. The goal of the item is to only be efficient very late game, and to allow an Armor stacked, or MR stacked tank to cover the other neglected stat, and as such, scale appropriately into late game.

Thoughts?

Remember, the stats suggested are just an example so you can understand the purpose of the item, the important thing is the item concept.