They could bring back the old Adaptive Armor: If you have more armor than MR, gain 4% of your bonus armor as MR/if you have more MR than armor, gain 4% of your bonus MR as armor.
And before anyone talks about Malphite/Rammus/Galio abuse cases, this mastery still wouldn't be broken on them. A Malphite with 640 bonus armor (probably the highest realistic level of any single resistance stat in the game) would only gain 26 MR. Which is still good, but it requires building full armor (Frozen Heart, Iceborn Gauntlet, Thornmail, and Randiun's/Sunfire/Dead Man's/Zz'Rot with Ninja Tabi, Scaling Armor runes, and Unyeilding/Legendary Guardian masteries, and all it really does is negate the flat magic pen from Sorcerer's Shoes and magic pen Marks.
So it's still a useful mastery, especially if the enemy team is running 3 AD damage dealers or 3 AP damage dealers, but it's not broken or oppressively strong. And it's also not that useful of health based tanks (Mundo/Volibear/Tahm Kench) or health based bruisers (Fiora/Garen/Darius), who will still benefit more from Tough Skin or Explorer (these tanks/bruisers being listed because their primary tank stat is health, not because they have no resistances).