AC is suppossed to reduce hits on your character, but in this day its more or less useless in high-end content for most builds.
So i was thinking - AC should scale at the same pace as To Hit do without making the character totally immune to damage. Since we have glansing blows and automatic hits on high rolls the later part is handled.
So in addition to the effect AC have now we should add the following.
I suggest that each level should have a AC rating baseline, that AC number should give the most defence for the cost involved. The benifit should climb fast to that baseline (AC of character minus the starting 10) and climb slowly after (diminished return for real high AC).
Let AC have a percentage change for a failed To Hit based on current AC, scaled to level.
Ex (the number here are not exact or even reasonable - only for showing a pattern)
Lets assume that we want a miss change of 30% for a 'good' AC for a given level.
[formula for miss/AC point): (baseline AC/10 AC points gives 3% miss change up to baseline +1% above the baseline]
Level 4: AC 30 -> Baseline AC 20 => 2 points AC above 10 gives 3% up to 30 then +1%. So 26 AC have 24% miss, 34AC have 32% miss, 42AC have 36% miss, 50AC have 40% miss, 70AC 50% miss and so on.
Level 14: AC 50 -> baseline AC 40 => 4 points AC above 10 gives 3% up to 30 then +1%. So 26 AC have 12% miss, 34AC have 18% miss, 42AC have 24%, 50AC have 30% miss, 70AC have 35% miss and so on.
Ofc 30% is taken form the air - 20% may be a more reasonable starting number.
Some benifits from this aproach:
No one will want to totally dump AC, getting it up to 'good' will be to benificial.
Feats like Precision can be made usefull if they reduce the miss chance on opponent, say +5 to Hit -10% miss & -5 damage.
Feats like that affect AC & ToHit (mobility/weapon focus/spring attacks) gan be given a set percentage on adding/reducing miss change, say mob +4%, W focus -2 % spring attack -4% etc.
On a side note: If fortification was changed - it could be linked to AC in a simular maner before other bonuses are applied.
Sorry for the long post and hopefully it is is somewhat readable and comprehensible.