* Armor should be suited to your character. Stronger characters can carry more armor.
* No arbitrary class restrictions ("wizards cannot wear armor", etc.).
* As your level rises, so does your AC.
* Armor should follow a simple formula rather than an arbitrary list not-quite-historical armor types.
Here what I can up with.
Light, medium and heavy armor are three different categories (light/medium/heavy), but with no fixed AC. One person's light armor is another person's heavy armor, etc. You can also be unarmored.
You can also have No/Light/Medium/Heavy encumbrance, but with no exact correspondence. For example, you can have light armor but be heavily encumbered (specially if your carrying lots of backpacks etc.)
Rounding: there is no need to round any numbers; it your strength is 15, half of it is 7.5, so 7 is smaller than 7.5, and 8 is larger, etc.
AC is 10 plus Dexterity modifier.
Anything that weights less than your one fourth of your Strength. The AC bonus is equal to the weight. You still get your Dexterity modifier.
Anything that weights less than your one third of your Strength. The AC bonus is equal to the weight. You still get your Dexterity modifier, but it is reduced by one third.
Anything that weights less than your one half of your Strength. The AC bonus is equal to the weight. You still get your Dexterity modifier, but it is reduced by half.
* None: If your current encumbrance is smaller than one third your Strength. You get a +1 bonus to AC, sneaking around, acrobatics, etc.
* Light: If your current encumbrance is smaller than half your Strength. You have a harder time casting spells. You might get an AC bonus if you're a monk, etc.
* Medium: If your current encumbrance is smaller or equal to your Strength. You have a harder time casting spells, sneaking around, etc.
* Heavy: If your current encumbrance is larger than your Strength. You have an even harder time casting spells, sneaking around, etc., and your speed is reduced.
|Weight (= AC bonus)||Dex modifier|
|Light Armor||smaller than Str/4||100%|
|Medium Armor||smaller than Str/3||66%|
|Heavy Armor||smaller than Str/2||50%|
|No encumbrance||smaller than Str/3||+1 AC bonus|
|Light encumbrance||smaller than Str/2||affects spells|
|Medium encumbrance||smaller than Str||also affects Dex|
|Heavy encumbrance||larger than Str||also affects Speed|
The results are pleasing, but not ideal. Take a look at this chart; it uses the rules above and assumes that "no amor" gives you a +1 bonus to AC (which is only true if you're unencumbered):
There are some problems. Medium armor is often useless if you have high Dexterity, for example. A simples fix would be allowing your full dexterity bonus to medium armor:
Anyway, it's a matter of fine tuning. And there is LOTS of fine tuning to do. For example, under this system you have good reasons to avoid wearing the heaviest armor you can.
But maybe even HAVING light/medium/heavy armor is one step too far. Encumbrance alone might be enough. Once again, this might be too complex for the types of game I play, but I cannot help playing with the mechanics of the game. But maybe I can come up with a mechanic that is even simpler. Let's see...