Idea for an Inspiration or Resolve-located tankiness rune
Masterwork Armor
All completed items with MR or Armor on them have their total cost (and final combine cost) increased by 250 gold (or somewhere in that ballpark). In exchange, each item gives an additional X points (a balanced number, for instance 12) of the respective resistance. For items giving both resistance types, get X/2 of each resistance.
A rune that lets people bend the rules regarding defensive item slot efficiency a bit (which is why I could see it in Inspiration). Essentially, it would allow their users to translate a higher amount of their gold into defensive stats - allowing them to be more tanky in the lategame. The trade-off is, like with the other scaling runes, the lack of early-game power, as the benefit only comes in when completing a whole item (that is more expensive to complete as well).
Example:
:
Without this rune, Sunfire Cape has 60 Armor, 425 HP and its passive for 2750 gold. With the ballpark values above, Sunfire Cape would give 72 Armor, 425 HP and its passive at a price of 3000 gold total.