The “criteria” for adding a civ to AoE2 are already established: it must be (1) a people or ethnic group (2) from the Middle Ages (based on the campaigns, roughly 400 to 1600 AD) that (3) formed one or more kingdoms. This has been the case since Age of Kings was released (with the sole exception of the Byzantines), even though the developers have recently tried to break them at every turn.
Consequently, the aspects you noted while relevant to gameplay are either already implicit (if by “political development” you mean having a kingdom) or secondary, given that every kingdom is presumed to have an economy. As for military systems, few are truly unique in history; most successful technologies, tactics, and styles of warfare tend to be copied. Isn’t that the reason we have regional units? Or why the names of certain technologies and unique units like “Stirrups” and “Composite Bowman” sound so silly? Given this, I don’t see how military systems can serve as a criterion.
Also, Europe was indeed fragmented and unique in its own way, but it wasn’t the exception. Various states emerged around the world, each with its own peculiarities. It’s just that, for historiographical and sociopolitical reasons, Western popular culture places Europe (or rather, specific parts of it) under a massive magnifying glass compared to other regions, though this is slowly changing as scholarship advances.
Consider this example and tell me if it doesn’t reflect what I said above: the Songhai Empire was a medieval empire with an estimated population of 40–50 million (80,000 in Timbuktu alone) and a territory spanning 1.4 million km², surpassed in medieval Europe only by the Byzantine Empire. Furthermore, its impact on the region is comparable to that of the Carolingian Empire, having influenced various areas such as the economy, architecture, law, politics, science, warfare, religion, society, and so on. This means it held immense historical significance. Yet, how many people do you see here on this forum or on Reddit, Discord, or YouTube suggesting their inclusion in the game? Or rather, how many do you see doing so as frequently as those who suggest splitting up the Italians? Don’t you find that curious?
I know you aren’t proposing we choose between Europeans and non-Europeans, but, as others have already pointed out, AoE2 has a limit, even if we don’t know exactly where it is. It can’t be expanded forever, and some interesting yet lesser-known kingdoms might simply miss the chance to be depicted and made playable, which feels unfair. That’s why most people were deeply disappointed with 3K; we might not get another chance to see the actual Tanguts and Khitans, along with the Tibetans, and that is infuriating.
@smiki5five I haven’t forgotten what I promised, friend. It’s just that I’ve been really short on time lately and a bit disheartened due to real-life issues. But I still intend to share what I discovered about Nubians (especially since I have no one else to share it with besides this forum lol).