linear tiers and direct superiority relationships are boring. i would like to see armor that is more of a collection of properties than a single value number. these properties could encompass protection vs different damage types (piercing, slashing, fire, etc), disadvantages due to encumbrance, how intimidating it looks, whether it will keep you warm in the snow, etc. distinguishing between different damage types increases the feeling of realism and makes the combat more strategic. for example, maybe you should send the guy with the plate armor against that pack of enemies holding swords while your nimble leather-wearer deals with the troll with the club. there's a whole world of strategic depth lost by having that single AC value.
the type of armor a character wears should impact how that character can (or must) be used during a battle. protection vs different damage types does this somewhat, in that it suggests different assignments for characters in a specific battle. another good way to do this is by having armor affect movement speed. with light armor, a character should be able to move faster and therefore have an advantage at positioning. however, if you don't take that advantage, your character will be hit more and generally would have been better off wearing something heavier. there's a great opportunity for dealing with character advancement here: your level 20 uberwarrior should be able to sprint around in full plate due to his legendary strength and endurance. this is a way better way to make him feel more powerful to play with than simply increasing his health and damage.
i would try to avoid overbalancing though. having every armor type work out to be equally viable feels flat and gamey. in a lot of real life scenarios, plate armor would simply be superior to other armors due to its protective qualities (of course in other scenarios, the encumbrance from it could mean your death). this is not really a problem though; if it's more powerful you can just make it cost more and be harder to find and give it an ongoing maintenance cost somehow. it would be great to be able to save up a bunch of money and equip every party member with heavy armor and have an unusually durable team. most likely this wouldn't actually be an optimal strategy though, as you could get away with having your non-frontline fighters wear lighter armor by keeping them out of harm's way and use the money for something better. basically the armor system would be implicitly allowing players to opt out of some of the positioning/microing gameplay by spending in-game funds, which is pretty cool imo.