I'd like to see armor handled in the same way that Master of Orion 2 handled weapons and special systems. In MOO2, older weapon types alwyas held specific niches in warfare, and even after you had discovered 2, 3, or even 4 generations of new weapons, you could still go back to older weapons and use them in useful ways (also, each new generation of weapons provided new buffs that you could add to old weapons.) So rather than have a linear progression in which each discovery out-dates the previous discovery, each type of armor would have special angles that would keep them useful depending on how you equip your armor.
For instance, your first armor type might be a simple leather tunic, which gives minor armor advances and allows your units to maintain a brisk speed. However, you might discover splint mail later which grants more armor but less speed. Discovering splint mail, though, would also let you add the "splint rib" buff to your leather armor, which adds armor to your leather armor (still less armor than your new splint mail) but raises the cost of soldiers created with leather armor. Also, resistance could be implicit in specific kinds of armor, rather than the player only being able to stack on resistance to armor types. For instance, "Feather Iron Armor" might give a soldier enhanced speed, decent defense, and resistance to lightening damage. "Magma Iron Armor" might give a soldier enhanced damage, decent defense, and resistance to fire.
What might be even more interesting are emmergent "special" armor types from novel techs. For instance, you might discover a small population of unique, giant, tame insects. If your farming techs are high enough, you can tame them and open up a research option that gives you "Chitenous Armor" made from their shed exoskeletons. This armor might be fantastic for the mid game, but gradually lose its edge as the game moves on. Numerous armor options like this could exist, not all of which are available to everyone (someone without the right farming techniques might not have the option of taming the insects and, instead, only be left with the option of slaughtering them or leaving them alone.
