The Perfect Tank Item

So I have read through a few discussion about the perfect tank item because at the moment there is nothing that perfectly completes a 5 item tank. I believe that from lack of great MR items, but regardless tanks need a item that can be the late game god tier item. I also understand that the item can't be to perfect that it makes bruiser and fighter to strong that they don't need other items to tank.

The Idea I have is to make a tank item which functions like the item would give X MR for every XX amount of MR you have. So maybe the item would give 25 MR for every 100 MR you have or something like that, or maybe it does that for Armor or even both MR and Armor but for less each like for every 100 MR you get 15 MR and every 100 Armor you get 10 Armor. The item could be build like or something like that and cost around 3k give or take. The item wouldn't give that great of base stats but would give that great passive which could be good later in a game as a third or fourth item as a tank.

This is just my idea, the numbers would need to be worked on but I think it could work. I feel it would be more fun for tanks to have item diversity. Thanks for taking the time to read.

-Itzmystory