Based on what they said, I think they are going to be making base resistances a bigger part in champion durability by changing defensive items into % multipliers if the item is intended for Tanks.
For example, we could see Tanks get their base armor increased and then:
Thornmail
+100 Armor -> +90% Base Armor
Randuin's Omen
+60 Armor -> +60% Base Armor
Frozen Heart
+90 Armor -> +85% Base Armor
And for magic resist (although I think MR itemization will be expanded upon overall):
Spirit Visage
+50 Magic Resist -> +80% Base Magic Resist
Banshee's Veil
+70 Magic Resist -> +110% Base Magic Resist
However, for hybrid items, the totals would be kept the same, so we would see items like:
...overall remain the same.