Underarmour

Think about it the better armour you have the harder it is to kill you and the fact its exponential makes it hard to justify using worse armour. So why not allow leather, chainmail, copper and perhaps even iron to be used as underarmour causing you to take more durability damage for more protection against high damage sources.

It balances armour progression, increases resource use (since both armours would take the durabillity damage essentially doubling the durabillity you could lose when getting hit) and balances mending in single player due to the fully repaired items returning xp to the player (4 more items that could have mending means 4 more chances for it to not work)

In multiplayer it would nerf all the end crystal tnt minecart and respawn anchor strats when used with the best armour and allow poor players the ability to challenge richer players if they risk more items. The rich players gain little by using underarmour against low damage sources since the armour would break before they died anyways, but others gain the ability to not die in one hit before they use thier items to fight back (its an issue as currently dropping even with e apples happens just because you dont have armour toughness from diamond armour).

This would balance the game for multiplayer (even more so now that half your slots might use pumpkin and elytra in the new ver), and also give uses to the low tier items discarded upon getting something better while still beeing usefull in singleplayer.