Another option would be to remove Armor from a defensive bonus to damage protection...you know....what armor actually does. This would give a DEF (since Armor Class doesn't really apply anymore) formula of:
DEF = 10 + CS + Shield + Misc
Lets go through the levels again and compare. Note, I'm doing short hand (Warrior = W, etc) and assuming the same set up as the previous post.
Level 1 - W 1 (12), D 0 (10), R 0 (10), M 0 (10)
Level 3 - W 3 (14), D 1 (11), R 1 (11), M 1 (11)
Level 5 - W 5 (16), D 3 (13), R 3 (13), M 1 (11)
Level 10 - W 10 (21), D 6 (16), R 6 (16), M 3 (13)
This is a little better, as level Divines and Rogues would need a 12, 13, 13, and 15 to hit at each of the levels. It's a little low(ish), but if I'm adding damage reduction via armor, being hit a little more often is a decent balance.
On the reverse side, Warriors only need a 9, 8, 8, 6 (and a bit less for magic-users as the levels go up). Which also isn't terrible.
These numbers are also assuming no class abilities or magic bonuses which would be too many variables for me to take into account.
Side effect of this, I could add the Non-proficiency penalty for attack rolls to your DEF when wielding a weapon you're not proficient in. Seems to make sense.
I don't see this as unworkable. It reflects "reality" while still being gamey and not complicating the rules unnecessarily.
Compared to the RAW rules....I think at low levels, fighters might be hit a little easier in this version, and Divines and Rogues (rogues especially) take a bit of a hit. As the levels raise, I think it gets close to what you'd expect for normal AC rules. BUT, I don't need any extra rules for Touch Attacks, as Combat Skill would still apply....although I guess shield wouldn't...or would it? Should touching a shield count as touching the person....I'm leaning towards no...
If it's not that much different once you start gaining levels, why not just use the rules as written? Good question. This version of defense reflect the character's growth, rather than their wallet. It also has the advantage of not requiring PCs to have to run around town in the armor while still being able to defend themselves. It also means that a higher level fighter will be able to hit and defend himself better against a lower level fighter, even if they're wearing the same type of armor.
But you said the problem with what you did in the previous post was doubling up on defense, relating to CS and HP. Isn't having armor as DR and HP basically the same thing? ...Kinda. The weight of heavy armor makes it something most won't wear without penalty (which I'll get into in a later post) and potentially reducing what you can actually accomplish in a combat round.