The main downside of armor is encumbrance; it slows you don't, makes you tired, etc. However, it is extremely useful in combat.
Now, nobody likes dealing with encumbrance. Okay, some people do, but it seems enough people dislike the concept since 5e added an alternative to encumbrance when describing armor: minimum strength of 13 or 15.
Which, curiously enough, seem be be pulled out of thin air, having no relation to the armor's weight:
|Ring Mail||30 gp||14||—||Disadvantage||40 lb.|
|Chain Mail||75 gp||16||Str 13||Disadvantage||55 lb.|
|Splint||200 gp||17||Str 15||Disadvantage||60 lb.|
|Plate||1,500 gp||18||Str 15||Disadvantage||65 lb.|
Also like the weapon's list, most of the armor list seems to be useless once you have a bit of coin to spare.
The ideal armor list would be concise, simple, but with enough nuance to provided cahrachters a lot of variation... not an easy thing to do.
But we will!
Here is the deal: you have as many SLOTS of encumbrance as your Strength score. Each slot is about 8 pounds, but this isn't meant to be exact.
Let us start with Strength 12 as our example.
You can add as much armor as you want... keeping a limit of +8 AC, like in 5e.
Let's try adding +4 AC:
You can still add your Dex bonus to AC, but for that you have to leave EMPTY SLOTS; one for each +1. Other slots can be filled with gold, water, weapons, etc. Probably two slots for heavy weapons, one slot for two light weapons (don't get me started on the meaning of the "light" and "heavy" properties).
Of course, you can combine the two:
And a shield! The shield is +2 AC as usual.
Of course, this is a bit prone to abuse and must be fine tuned.
One would think that in most circumstances PCs would have little space for abuse, since everybody will be carrying other stuff anyway.
Maybe a strong barbarian could carry lots of armor... but armor is of very limited utility to barbarians anyway. Other classes do not benefit from having BOTH dexterity and strength.
And with that, we get rid of the whole armor table, and replace it with this:
Light Armor: +1.
Medium Armor +2 to +4.
Heavy armor: +5 to +7.
Cost is the AC bonus, squared, times 10 gp. So, 10, 40, 90, 160, 250, etc.
Clumsy armor halves the cost (disadvantage to stealth). Any armor with AC 15 or more must be clumsy.
Fancy armor allows you to add +1 to AC to the maximum but quadruples the cost (i.e., AC 15 medium armor costs 500 gp; start with 250 gp, multiplied by four to 1000 gp, but halve that cost because it is clumsy). So you can have +2 light amor, or +8 heavy armor (640, divided by two, times four... 1280 gp).
Nonmetal armor is limited to AC +2.
Now that you got the idea, you can play around with it. Let anyone that has less than half their slots filled get an automatic +1 to AC, for example, or take a -1 AC penalty if you are carrying more weight than your slots would allow. Maybe require empty slots for AC bonuses from Wisdom or Constitution, forcing the monk to travel light (which is probably too harsh) and avoiding abuse from the barbarian. And so on.
Minor update here.