What if instead of discovering better weapons through the tech tree, weapon, armor and buildings were developed through a process similar to the dynasty system. 

First there are simple improvement lines in the military tech tree that starts with the basic weapons, basic armors and basic military buildings. When you research the first lvl you get all the basic forms and choose a focus for your faction. This focus lets you customize as much as possible the appearance of a single type of armor, type of weapon and type of building.

Every level of tech after that gives you a +x to various items/buildings (like you have in age of empires or Warcraft) and perhaps unlock a 2nd or third cultural style.

At this point nothing is different from any other faction about your W/A/B except the look and your + bonus level. However as reward for quests or found on monsters are scrolls of "lost lore" that teach you bonus/abilities you can add to your cultural W/A/B. Then through diplomacy you can "mate" one design with another factions creating a new generation of W/A/B that combines the benefits and appearance used by both factions.

Finally there would be a negative hidden trait that only become known after say 50 of the item are produced. These might inspire the faction to seek another mating with a different design in hopes of a "better" negative trait.