Wow... what a mixed set of reactions. Congrats on making such a great case... you almost bought me in.
NO. I would have to disagree, unfortunately, for my own reasons, as everybody else did!
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RANT: the OP shown some classical western european pre-conception: not much is known(?), yeah right maybe about the politics in Europe!! (as state level forms of manifestations were probably inexisting until after the 800 - so you basically have a lot of tribal/gang like warbands kicking ass - SO NO, THE MAP IS NOT COMPATIBLE, for western europe city states are more recommendable than regional powers like counties/duchies); otherwise, muslim world was flourishing at that time and the byzantine had a continuous existence. Sure, some information got lost and IMO is not enough to make a game about this...
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I subscribe to those who thought that such a request is best suited for a remake of ROME!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
PLS remake ROME first (the work done on CK2 and the new EU4 should help Pdox get great new ideas to add to Rome as well... would definitely want to play the improved version). The Dark Ages proposed here would fit so well into Rome (we can have stuff like people's migrations and how the BE coped with it, the formation of new states with the effect of barbarian migrations and MAYBE migration of people that didn't do so in history - under the fear of barbarians migrating to their own lands :d)... whatever. lot of ideas, but I would like to play an improved Rome much sooner than just this.
Also CK2 is not compatible with the Dark Ages in terms of armies and units, but a Rome continuation would be close to it (at least for states that survived the fall of the roman empire).