I would like to see a System, where heavy armor makes it more likely to get hit, but mitigates most types of damage. The way i see it, somebody in full plate is not going to be able to jump around and evade hits.
I generally like the system of D&D, with the different damage types, especially since they could be used to determine a weapons effectiveness vs armor. However i feel, that armor penetration as a stat on weapons is probably the better and easier to balance approach.
Shields should mostly change the chance to be hit, but some weapons such as crossbows and maybe axes should be able to do damage on a successful block. Possibly something like when a crossbow bolt gets blocked, the armor value of the shield and the worn armor is applied to the damage calculation. And when it is not blocked only the worn armor is considered in the damage calculation.
Penetration for melee weapons should look different. Axes and Hammers should have a small chance to penetrate the shield and cause an injury to the shield arm. That injury should lower the chance to block.
Armor should also effect Magic, for example lightning should be more effective on metal armor, while short duration fire or ice should be less effective. Physical projectiles summoned weapons created by magic should behave like their normal counterparts.
Lighter armor should allow for more avoidance, some evasion feats or skills should not work with the heavier armors.
The balance between those should shift the heavier the armor gets. (Ideally i would like to see a system that checks what kind of armor you are wearing on which body part and calculates damage based on the body part hit and the armor worn there. But that would be too hard to implement and balance.)
Armor should also have an influence on certain combat related abilities like sneaking, knock down/stun chance, time to stand up after stun and such.
If a rouge in leather bull charges a warrior in plate armor he should have a much smaller of knocking him down compared to the reversed roles(The rouge could also evade the knockdown entirely though, so that balances it out again). The rouge should be able to get up again much faster however. The knockdown would strip away the rouges defense since he can't evade anymore, while the warrior keeps his defense. If they spent equal time on the ground it would either mean a death sentence to the rouge or rather meaningless to the warrior. If different times on the ground don"t work then maybe it could be something like having a higher chance to be critically hit the heavier the armor.