Instead of simple +1 Attack/Armor upgrades I would prefer some sort of bonus you decide between on top of the extra damage/armor. Similar to how you advance age you develop different types of armor and weapons by deciding what type of weapon and armor you will apply to your units every age. Each Civ can have their own unique ones. The different choices are categorized but open and should be more of a bonus aimed at making your planned tactics stronger instead of locking you out.
Example on England:
Upgrade 1 Ranged Weapon:
A - (Defensive Weaponry) Bonus Range and Damage for Defensive Buildings, Bonus Damage for Villagers against Animals.
B - (Fire Arrows) Bonus Damage for Archery Range units against enemy buildings
Upgrade 1 Melee Armor:
A - (Inspiration) Bonus Defense for all nearby units during Network of Castles
B - (Lighter Armor) Bonus Movement, slightly cheaper units
C - (Heavier Armor) Bonus Melee Armor, reduced movement