We have options for AP, AD and Armor, so it stands to reason that we could round out the set with an MR item. With a lot of people complaining about Mordekaiser's performance with Conq (myself included) it would give tanks a way to lane against him, since most of them can't build Bramble Vest to mitigate his damage when he's AP and only seldom autoattacks, and also usually can't build Executioner's or Morello without trolling.
I feel like an MR version of Thornmail that reflects damage based on ability hits would be cute, but there are a lot of other ideas that could have potential. A Grievous Wounds version of Abyssal Mask that gives less stats is also an option, or an MR build of Bami's. It's something to think about at least.