Bumping this thread as I just started playing the game in earnest.
First, to the Devs, I have been playing Civ for over 20 years and this game enthralls me in a way that Civ hasn't always (still love Civ of course). What drew me to CK and EU draws me to this game too. Bravo, really. A great basis for a game.
I absolutely think expanding to the Middle Ages is warranted. Late Antiquity had just as much family dynamics as Classical Era. Sometimes moreso. I think that to enable this, a mechanic should be introduced around the idea of an "Empire" which isn't currently in the game, starting at the beginning. It should be a threshold that you work towards and then you implement certain decisions to form your nation into an "Empire" as long as certain criteria are met (should include size, military, tech). (Think forming or re-forming dead Empires in EU). The premise should not change the game much in the current era, but would matter toward the end of what we now consider a full game. The other players can also form Empires but it should be difficult enough that it is self limiting to achieve even on a huge map. (Think Rome and Persia - only two real "Empires" of the era). Once you create an "Empire", there should be an added "administrative region" layer that can include several cities and have it's own seperate Governor. There should be a limit to the number of regions, and a region should include some added minor bonuses. It also changes your leader to Emperor and add a new set of "advisors" called imperial guard that you can appoint. The loyalty of these guards will be important and if they drop too low can cause negative events like your leader getting killed and replaced with one of them or another choice. Loyalty of territorial governor's will be important too as they can rebel and take several cities with them.
Eventually, to enable the transition to middle ages, there should come a point where (maybe with a certain tech or being an Empire as a trigger) a combination of Tribe migrations start appearing and loyalty decay increasing in provincial governors as losses mount. (-40 Opinion, Fighting in Governed Province). You can appease the waves of barbs with treaties to give them cities as vassals or let them join your armies, etc but these will come at a cost of increasing risk of bad events (army revolts, cities declaring independence). Giving a city to a tribe within a Governor's territory should significantly harm their loyalty, as well. Letting them in your army should hurt your relation with the imperial guard and also increase the number of full-blooded tribe-folk in the pool to select from for those positions with less loyalty to you. You of course can also choose to attempt to keep fighting at great cost to your empire which may eventually spur your Governors to declare independence...
Anyway, the goal would be fractionation of the larger Empires to create a "middle ages" effect. Those who remain will come out very changed in both population and societal structure.
Anyway, lots of nonsense here butnthe goal is to not take away from the early game and respect the game in it's current state, as the goal is to still build up and expand to the Empire we all want to be, it can just add a really cool "late game" dynamic that can keep the game fun after you already feel like you won the first portion.
Just thinking out loud!!