Two things: I think it would be good if there were more types of shield available, and changing the distribution of shield proficiency so that it isn’t basically synonymous with medium armour proficiency
Let shields be more than just a flat + to AC unless you’ve taken some specific feat or class feature, let there be things like:
Spiked shields that deal small amounts of damage when you block a melee attack
Magic shields that can block or are especially effective against spells
Throwing shields that can be sacrificed in battle to used like a ranged weapon
Specialised shields that are more effective against some types of damage but more vulnerable against others
Shields that aren’t a flat bonus but boost your AC off your INT or WIS mod or half-proficiency bonus to show how a certain amount of skill is needed to use those particular shields to their potential
And I think it makes sense letting shields be their own thing rather than part 2 of medium armour,
Maybe the fighter and paladin gets heavy armour and shields but cleric only has heavy
Maybe Ranger only has medium armour
Maybe Bards and Sorcerers have light armour and shields