I agree, rather than going with a +1/+2/+3 escalation, different quality levels of armor could be introduced.
I was thinking, on the Chainmail/Scale Mail question of how do you make that stuff scale (pun intended) to end game?
What about giving consideration to materials alongside craftsmanship?
I remember how different and interesting the Ankheg armor in BG1 was compared to just plate mail sets (and the unique benefits it gave for druids etc)
Scale mail could graduate in effectivenes depending on what it's made out of?
Rarer scales from dragons could give great protection and color specific benefits, and other lizards (basilisks etc) could have resistances or quirks.
For chainmail, different types of ores, metals and their varying properties or rarity could come in to play.
(the obvious fantasy option here being mithril) but you could go with meteoric steel, adamantine etc etc.
the same would obviously apply to scale mail made out of metal.
If there was any kind of crafting system, this would then open the door to plenty of interesting possibilities to get this stuff made yourself,
or just quest for it or find it in the world.