Shields might be handy AC boosters. But as armors are almost unbreakable what about shields? They might break.
When character or a creature with a shield is hit with natural 20 result, there's a change that the shield breaks.
4 in 6 change for small shields and bucklers
3 in 6 change for medium and wooden shields
2 in 6 change for large and enforced shields
1 in 6 change for tower and metal shields
If there are no different types of shields in game use fifty-fifty (3-in-6) or what is most logical.
Also for flavor if an enemy attack is strong enough (lots of damage with a single blow or several attacks in a row for example) the shield might break. Use shield's change to break or x-in-6 change what suits best.
Sometimes natural 20 isn't necessarily needed at all. If a dragon stomps the character with a shield rolling well but not well enough to hit or rolls ridiculously bad damage, maybe it was the shield what saved character this time breaking into splinters in process!
In my opinion shields can be much more flavor and even character's trademark than just a static AC bonus.