In the draft for Hack'D & Slash'D, I have armor class as a negative value you subtract from damage. In theory, I like it... in practice, it kind of sucks. If you deal 4 points of damage against a foe in decent armor, you might end up dealing no damage, or 1 point, which is sort of discouraging. It also requires me to tamp armor class down... a -5 is basically game breaking.
However, (as I started to suggest last post), I could swap it so that every time you are hit, you check your armor class. If you roll your AC or lower, you sustain half damage (sort of like a check to resist magic). That's reasonable, and fits with other mechanics.
So, leather might give +1, while plate mail gives +5... so wearing a suit of plate mail +2 gives you AC 7. Any time you are hit with a physical attack in combat, you check your armor class (roll 1d20, wanting to roll 7 or less). If successful, you suffer half damage from that attack; if you fail, you suffer full damage. Armor then doesn't totally neutralize damage, but it's always helping you (or potentially helping you)... and even an armor class of 10 (which would be very, very rare) only cuts damage in half 50% of the time. So, it's a nice thing to have, and it has an ongoing effect in play, but it doesn't break anything.
File that under LOVE IT.
Oh, and shields then grant you advantage on a number of AC checks each round. A small shield would give you advantage on one check, a large shield on 2, while a large shield +3 would grant you advantage on up to 5 armor class checks per round (which would probably just be all of them all the time).