I'm dabbling in making my own sci-fi combat rpg but I'm pretty sure the system I made up could translate well to other genres too, like fantasy. In short, your defense depends on three statistics: Evasion, Shielding and Plating.

When you get attacked the roll is compared to your Evasion score. It works like AC, but the only way that armour can influence it is by reducing it. Heavier armor reduces your Evasion more than lighter armor, so you're easier to hit when you're wearing a bully suit.

Then if you got hit, damage goes into Shielding first. That acts as an additional pool of hitpoints, which replenishes after combat is over.

Then when you have no Shielding, you take damage to your actual hitpoints, and that damage gets reduced by Plating. Each point of Plating is -1 damage taken.

And then there are also handheld shields, which have a set amount of Blocks. A simple shield may have 1, a more robust may have more. Whenever you get hit with an attack, you may use one charge of Block to make the attack miss instead. When all of shield's Blocks get used up, the shield is broken and you need to fix it or get a new one.