Tie this to a side quest unique for each map. Here are just a few ideas for elemental armor:
Orange – Armor repairs automatically
Green – heals the player every time a plate breaks
Purple – Mitigates 100% damage only from the back, shadow rifts zombies when a plate breaks (2 plates)
Red – Zombies that hit the player's back will have a window's wine effect
A bonus would be having armor look unique when worn by a player, i miss having extra items that you can build to setup on a map.