for me
I'd be satisfied with this type of spread, comparing values of rogue guy with 20 dex vs plate guy with no dex
Light:
Leather / Studded leather (same thing! i.e. get rid of studded leather, make the studs have a combat usage, like damage when you get grappled) : 11 + dex = 16
Hide Armor/Chain Shirt = 12 + dex = 17
Medium :
Chainmail : 14 +(2 dex) = 16
Elven Chain (considered magical, no +1 needed) : 14 + dex = 19
Breastplate : 15 + (2 dex) = 17
Splint : 16 (no dex)
Heavy :
Scalemail : 17
Plate : 18
Mithril/Adamantine/Dwarven Plate : 19
Full Plate (get rid of field plate) -- extremely rare, costs 20k gp even for mundane : 19
Mithril/Adamantine/Dwarven Fullplate : 20
*note : Elven chain is still medium to my mind, for feats, but allows full dex. (so, AC 19 with 20 dex). Its equivalent value would be that of full plate +1, or maybe adamantium full plate or something, so for the same price you are still 1 AC ahead in heavy armor (fullplate +1 = AC 20). You can see how everyone would still want to own some elven chain...you can RP that the feat cost to use it is to maintain its sheen or it loses its magic. There would be no such thing as "elven chain +1", to me that's redundant. I'd rather "mithril plate" = AC 19 (like plate +1), or mithril fullplate = AC 20. Something like that. Forget the +1s on magic armor...it robs it of flavor in the flat math system. A +1 sword should not be in an arms race vs the +ses on magic armor. magic weapons should help you hit more, and not get into an arms race with the armor stuff, beyond a certain point. We all know how boring long combats can be with attrition where nobody can hit the other guy.
The idea is, unless you have 18+ dex, you will always be better of AC-wise wearing medium armor. If you have a 15 dex or less, you WANT to be in medium armor, at least. But you may avoid spending a feat for heavy armor until you find or can afford some plate. But that's if you don't mind clanking around the dungeon and falling out of your sky cell when the light armored guy trips you into the pit (go Bronn!)
And plate is ALWAYS better for defense than any regular guy could hope to achieve, no matter how fast he is, in light armor, unless he's magically super-fast. This is the balance for three feats, and disadvantage, and the costs, and myriad other ways plate armor can get you killed. (drowing, falling off ledges, breaking the rope bridge and falling into lava pits, whatever)
Full plate should remain an option which is extremely, EXTREMELY expensive. Let's differentiate thick hide armor and chain shirt not by their AC, but by other things such as heat to wear, which would make you prefer one type or the other depending on the clime of the local environs or the season.
Plate must always grant better AC than any (mundane) light armor or medium armor, even with 20 dexterity.
This is not that hard to do, if you combine / get rid of crappy options that nobody used anyway. Scalemail would be popular as a starting heavy armor until you can afford plate, around level 4 or so. Full plate should maybe come around level 8, if there is such a thing as expected wealth per level.
It's only natural if you have three feats to use armor, those armors should have a little breathing room to get better as you can afford more. I *like* finding magic armor, because regular mundane plate armor is so d@mn expensive. If they get rid of expected wealth, but balance the actual cost of plate, with a DM recommendation to dole out enough GP around the party to make its purchase take a while (i.e. not at first level...).
The only guy who should be in plate armor at first level is the paladin. Not because he's rich, but because it's his schtick...his order fashions one for him, it's a status symbol and represents his class. A fighter would be typically in chainmail or splintmail or even scalemail, depending on his Dex, at level 1. If he wants plate, he's gonna have to pay the price. Whether he pays the gold price or the iron price, it's up to the Fates to decide which...
