Is there any other way we can approach this, besides fiddling with numbers? A change in how they behave, or something?
Lets me think. Disregarding gloves and boots, we have:
Cloth armor: (Effectively) no penalties, low weight, poor-to-moderate protection
Chain mail: Moderate penalties, moderate-to-low weight, moderate-to-good protection
Plate armor: Substantial penalties, high-to-moderate weight, good protection
AoGH: .... Everything is bad, nevermind
Instead of taking away from plate, is there anything we can add? Something that isn't as easy to compare? Deflection chances, elemental protections, etc?
What about an armor proficiency skill, with unique benefits depending on the specific skill?
Currently every armor has some level of penalty, some amount of weight, and some degree of protection. Generally, higher AV comes at a cost of weight and dex/AGI penalties. I feel a bit like if all three armor types performed in a perfectly balanced manner in these three categories, the game would become... more balanced, sure, but also more bland. It would be interesting if choosing an armor type was a deliberate thing, something you could consider the options, weight the consequences, then pick one based on whatever criteria was most important at the time.
Let me paint a hypothetical example of a system. We'll say that we still only have 3 viable armor types. Chain mail, being the most moderate of the three, offers a great balance of weight-to-AV with reasonable penalties. No major drawbacks. But no major benefits, either. It's main draw is economy: the best raw AV for it's weight. A PC skilled in mail might be able to reduce it's penalties to virtually nil. By comparison, we have light armor (which would include cloth and even bare skin). These have quite low weight, and effectively zero penalties, but don't really provide much meaningful AV either. Their advantage comes entirely with skill: a talented light armor warrior could gain increasing chances of completely avoiding attacks - including traps. Lastly, would be the heavy armor group. High weight, moderate penalties, and excellent AV. Not really economical in the short term, however. But if you are strong enough to wear it, and skilled in it's use, you gain not only good protection against physical attacks, but are able to apply a larger amount of it's AV to fireballs (and lightning bolts?). So which do you choose? Some chance to totally negate traps and attacks? Better protection against the elements? Or sheer, balanced, physical protection?
Totally hypothetical, though. It's 2:30 and I'm hardly in a condition to be coming up with good game design ideas. Think of it as a springboard, if you want to discuss further.
"Put more stuff in the... thing where... more stuff goes in."