make a special Military Order mechanic where military orders you form can turn into full fledged civs of your own. they start out as your vassals and are formed for a special purpose (giving you a bonus to something), but can flip away from you if you do enough things they don't like
Let me 'double down' on this with a vote for more Non-Civ Factions or what is now called NGO (Non-Government Organizations) but in fact dates back a lot further and so could be implemented very early in the game:
1. the 'crusading' Military Orders - which in fact were almost completely independent of governments from the start, which resulted in pretty bloody government destruction of some of them (Templars, Teutonic Knights) in the end.
2. International Mercenary Companies - some of which, like the famous White Company, were army-sized organizations in the Medieval Era. Earlier, Xenophon's '10,000' from the
Anabasis was, in fact, a large mercenary unit hired by Persia - which, to be sure, was the only organization around with the money to hire a force that large, but it shows what could be included even in a Classical Era setting.
And, of course, another 'variant' would be the Industrial Era Foreign Legions, which included not only the French, but also a Spanish Legion and the mercenary/foreign contingents in pre-French Revolution armies in Europe: including the Irish Brigade in France, the Scots Brigade in the Netherlands, and the Swiss nearly everywhere until the Bourbons in France bought up a monopoly on them.
3. International banking family organizations in Europe at the end of the Medieval Era, morphing into the giant International banking firms in the Modern Era or the slightly earlier National Banking organizations (Bank of England, Royal Bank of Scotland, etc) and international Exchanges (Amsterdam, London, New York, etc) that attract money from outside the Civ and, frequently, send money outside the Civ to finance developments elsewhere - a huge percentage of railroad construction outside of Europe in the 19th century was financed with European or American money that way.
4. Other 'Religious' Groups (in addition to or a sub-set of the Military Orders) like the
Hashashin or the Shaolin with potentially Un-religious effects not exactly military but potentially disruptive (example: Hassan's merry Assassins removing your Great People or Governors now and then)
5. International trading organizations. As far back as Classical Athens there were informal groups of investors that provided 'insurance' or financial banking for international and long distance (by the standards of the time) Trade, and even when governments tried to control that (East India Companies and similar in the early Industrial Era) there were also 'private' firms independent of governments that greatly influenced developments: John Jacob Astor, Vanderbilt and the other Railroad/shipping magnates, etc.
Some of these 'NGO' groups could be handled by a Great People mechanic - Great Generals that form Mercenary Companies or Great Merchants that form independent Trade/Finance organizations, for instance. But in general, I think the game would be much richer and less predictable with more 'independent actors' in-game, and a group of NGOs would be one such set.
