But the game's symmetry also depends on how players can play the civilisations. Directly modifying the civilisations to push them towards more distinct playstyles than they currently have would, in my opinion, be the first step. Today, by sticking to roughly the same playstyle and the same build orders or unit orders, all civilisations remain generally playable. If we play the Carthaginians with a more Gallic playstyle, the civilisation remains fairly competitive. The only real differences between the game's civilisations at the moment are different but rather abstract bonuses (especially for new players), different textures, and a few unique buildings and units that don't fundamentally change the playstyles... In my opinion, we should (still) preserve 0 A.D.'s origins (AoE2) and draw inspiration from it. In AoE2 (or even AoE4), civilisations have things in common but still retain different playstyles! I haven't tried DE, so I can't say whether this mod adds differences between civilisations and meets my "criteria". What could make a difference, and what I also feel is missing, is the diversity and depth of Petra's playstyles, and I completely agree with you on that! But yes, changing playstyles depending on the civilisation would make balancing more complex, and I don't think the developers would put in the time needed to do that at the moment. Is there a mod that has experimented with changing AI playstyles? Anyway, we're starting to drift away from the original topic of the discussion 

I also find the game too symmetrical. What I believe needs to be changed and what I have experimented with changing is the behavior of the bots: small civilizations or vassal states should not act like great powers, and nomadic civilizations should not behave the same way as sedentary ones. The issue, however, is that such changes would make balancing the game difficult, and many players likely wouldn't appreciate it.

What do you mean? This is a mod for modern warfare; the bot will be adjusted accordingly. I’ve included drones in the mod and have already showcased one; there will be many more types, and they will be a key focus of the mod, alongside complex air defense systems. Yeah, I have thought about this.

Building a Market unlocks training Slave class units at the Farmstead and Storehouse. 20 max, but +10 per Market (this is the limit I'd like to make global). The Slaves are capturable and die easily, but are 2x better gatherers than Citizens. Slaves are a gatherer unit only, while Citizens maintain the ability to build. There are 2 technologies specific to Slaves: Slave Trade (-50% train time) and Slave Markets (remove training limit, but Slaves are double cost).   Citizens also have the "Slave Ownership" aura which boosts nearby Slave effectiveness. This basically replaced the old "Allure" aura of the old Female Citizen units.  

"Say hello to my little friend".