What stopped Roman empire from migrating? The fact that it was very hard to pull off, because they had gotten used to staying in one place. Bear in mind that demesne limit was only a widely understood mechanic to keep us on the same page. I used demecne size to reflect administrative capability.
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I think that different tribes could spawn spontaneusly. Say if Finno-Ugric culture group has a finnish county that goes over the population limit of the county. This county should spawn as a playable tribe that can start migrating. Similarily anywhere on the map.
I can't logically picture the Roman Empire picking up everything and moving somewhere else, especially when being conquered by barbarians.
The entire glory of their empire is based upon their might, which would definitely be questioned when they run away from their homeland, their history, which would seem much less relevant if they move somewhere else to start anew, and their architectural achievements (considering their military achievements would be falling apart at the start of the game), which I doubt they would abandon.
The difference between a classical era empire and barbaric tribes is that tribes live off of the land while empires make the land their own.
Also, I realized something thanks to your idea at the bottom. Tribes that were too uncivilized to be barbarians wouldn't just be nomadic or migrating everywhere.
Making them just spawn would make more sense, but people in Iron Man mode shouldn't be restricted to the Western Europe, Mediterranean, Middle East area at the start date. Maybe we could have another type of government called tribal that cannot migrate at all.
If they make a really good mathematical model of plague mechanics that takes into account trade, climate and relief, it can be reasonably accurate for the spread of religions, languages and cultures.
I would really like this to be a huge focus. I'm always interested in this part of history and I'm disappointed that Paradox doesn't represent it in most of their games.
Considering this shaped a lot of history (even more so during the Dark Ages), I think this would have to be implemented.
Feudalism shouldn't be some hard, fast thing like a government change in Alpha Centauri. Feudalism should develop naturally, as military pressures demand someone closer to the ruled lands who can more effectively drum up military units but who demand more freedom from you in exchange. You can, say, start out bureaucratically administering the heartlands and have governors in the outlying provinces. These governors can be given more and more control over their provinces, increasing their military obligations to the state each time, until eventually you've lost the right to even appoint the governor's replacement (let alone recall him) because the title is hereditary. These governors, now counts, can then become too much to manage, so you pick your favorite and give him control over some nearby counts you like less with the promise that he'll keep them loyal and deliver their forces to you when called upon.
The game rules need to allow gradual change in response to adversity, because it's my understanding that that's what defines the dark ages.
This, combined with BaronIronmaggot's comment about tribes, gave me an idea.
What if countries had, for lack of a better word, some sort of tree for their government type.
Tribes could become either Nomadic or Barbaric after passing some sort of threshold (based on technology?).
Nomadic tribes could settle and become Barbaric. Barbaric tribes could choose to become Nomadic (Each would cause some sort of administrative penalty, and if your people want to continue to migrate/remain settled, they could demand independence to do so. If you win, their population remains under your control but if you lose, they will become independent, gain a different name and culture, and continue to be nomads/remain settled).
Barbaric and Classical nations would then be able to have decisions where laws are made that give bonuses (such as more manpower or raisable troops (depending on how feudal you are)) but drive you further towards feudalism.
Here's a question, a game like this, should it be more national perspective (VII, EU)? Or character oriented perspective (CK)? Or perhaps trying to go for a mix like Sengoku/Rome system (which I don't think was done well).
I can imagine them doing character oriented and then adding in lots of chances to remove yourself from your liege to get huge army bonuses (manpower, etc.) to move into other lands. But then that just turns the game into chaos (and not really fun as then you get attacked by armies twice your size randomly despite being a pretty large country. I guess you can add some fun mechanics and choosing to become nomadic yourself ala Venetians leaving Italy to the Lombards and the Britons going to Brittany). And how would the splitting of the Gothic tribes work (actually this is something I always wanted in CK2- making cadet houses), etc.?
I think it should be mixed.
Important factions (royal families and (possibly) cultures and religions) and people with land (governors and kings) would be represented but it probably shouldn't be as deep of a system as CK2 for the issue of technical restraints (dynamic cultures, religions, population percentages, and constant changes in borders would be very demanding already).
I think a character who is independent, not the head of a royal house, and having a certain amount of prestige should be able to make their own royal house.
A mix that contains both elements. CK II like features for feudalistic regimes and EU like features for imperialistic-administrative-bureocratic regimes.
I think this would be most notable in political and military-based gameplay aspects.
