There are times when I wonder why I bother putting heavy armor on a fighter, paladin, ect... at some point it really doesn't make a difference anymore unless you put a major effort into raising your ac. You see many builds opting for the dexterity route ditching heavy armor. I would like to see a change made to make armor "useful" again. It seems to be it should be able to give more benefit than it currently does.
One Method would be to have armor give fortification based on the type. Light - light fortification, Medium - Moderate fortification, Heavy - Heavy fortification. This would give an immediate benefit to wearing heavier armor, though at higher levels this isn't that much of a benefit aside from possibly allowing the use of slightly different gear. It would benefit lower level characters more which isn't really where the problem lies.
Another option would be to create enhancements to for fighters,rangers, paladins and barbs to better use their armor. Though this might be a problem for some AP starved builds, I'm looking at you pally, It would allow the benefits to be added at more appropriate levels. I would envision, a percentage damage reduction to physical damage for wearing an armor type.
An example might be:
12th melee lvl: Armored Warrior 1 - light 5%, medium 10%, heavy 15%
18th melee lvl: Armored Warrior 2 - light 5% + light fortification, medium 10% + moderate fortification, heavy 15% + heavy fortification
In theory this would stack with shield mastery, though some might think that is over powered.