Great Invasions was so complex and so hard (especially with a bugged tutorial and no manual anywhere) that I still don't know how to play it, although it looked like very detailed.
It could (and should) be an interesting game to play and should be very different to a lot of historical grand strategy. It's not too complicated a period to portray acurately (the peiod was complicated, but then, so is all of history), but there are a couple of problems related to doing so:
It would be definitely interesting, but note this: like Crusader Kings' engine being and game system being different than that of EUIII, this "Paradox Great Invasions" would have to use a different game system, a system that allowed to represent several different kind of States: empires (Rome, Carlolingian), federal monarchies (the first Frankish kingdrom), military monarchies at the Germanic way (Visigoths, Vandals, etc), and, most important, migrating hordes (Burgundians, Goths, Franks, Vandals, Suabians, Alans, Huns...).
Great Invasions had something like it, but Paradox can surely do better. There should also be a process, in which an invading nation would start behaving like a horde. This could give them some characteristics, like the ability to
confederate with an empire (the Roman
foederati) taking some of his frontier land, but being able to rape this empire when it became weak... just like the Visigoths did with Rome. This could also have happened with the Sassanids, for example, or with the Carlongians, had they used the same tactics with the Magyars, instead od sucessfuly fighting them.
There should be some kind of indicative marker, because this game would be enterely about the authority of a monarch. Total decentralization would mean a confederate tribe in the Germanic or Hunnic way, and yet it would not be totally bad, because it would enable decisions for hordes and migrating peoples. When a country had a low authority marker (let's say, from 10 to 0), nobles would become more and more independent, taxes would lower a lot, ability to recruit armies would decrease... and there would be several uprisings, death and succession of a monarch could tear the country apart... and if this country was delcared war upon, it would most certainly fall fastly thanks to some events like "the nobles of this province are pacting with the invaders!". That's the situation in the Visigoth Kingdom of Toledo when the Arabians declared war.
After this invasion, there would be a chance for the invaded kingdom to "implode" into several successor states (in the case of the Visigothic Kingdom, that would be Asturias, Septimania, and some other pyrenaic domains).
To make it in-game, the Visigothic Kingdom had an authority marker of 3 or 4 at the moment of the Arab invasion. After the defeat of king Roderick in Guadalete, several provinces began to surrender (a choice of the Gothic lords to keep their sovereignity and convert to Islam, like the famous Romano-Gothic lord Cassius did, becoming the later bereber dominion of Banu Qasi, the
Sons of Cassius, and the Visigothic Kingdom, invaded, with no army and collapsed, imploded, and the successor states made peace with the Muslims.
Secondly, a major theme of the period is the rise and FALL of empires. The only empire to last the whole period was the Byzantines and that was a close run thing
Well, yes but not accurately speaking: the Carlongian empire survived, devided, but survived. It was divided between West Francia and East Francia, later France and Germany (and Italy and Burgundy, which became testimonial, but could have become important had their kings managed the situation better).
(and the empire in 1066 was very different to the empire in 476). The problem here is that people tend to like to play games they can win. Few people want to play games where they are almost certain to die eventually (of course, some people love them). To be successful, a game set in this period would need to either make survival and expansion over the long term achievable and risk accusations of historical inaccuracy, or define other goals for the playable nations- probably just survival for a lot of them.
There should be a way to show those political and social changes. In the beginning, IVth Century, there are two very different ways of government: the imperial one, represented by Rome and the Sassanids, and the Germanic one, based on a military assembly ruled by a rather weak king, who had only powers in war times and most of the times was elected. In the end, 1066, Feudalism is already formed, so we still have two types of government, evolutions of the former ones: The imperial one evolved into the
autocracy, to say a name, which is the Byzantine and the Muslim form, based on civil government. Different decisions, such as the creation of the Thematic system, or the Abbassid version of it, called
Iqtá, consisting of Turkish warlords being established in the frontiers (we've seen it before, it's like Roman
foederati), are repeating the same process before. Turks would arrive at early XIth Century as a migratin tribe, like Khazars, Pechenegs, Alans...
The Viking invasions would be somehow tricky to represent, and considering the sandbox nature of the game, every invading people would be able to represent those Normand invasions when they arrive to a Feudal country.
To keep it more or less historical, there should be some restriction to evolve into an empire. For example, the Franks did it because they used the former Roman administration, they had resources and the structural cohesional power of the Church. The Goths did not have this, they kept their aristochratic system of elective monarchy and didn't take advantadge of the former Roman administration or political system. They kept Roman law for Romans and Gothic law for Goths, as the Longobards did in Italy, or the Ostrogoths before them. The Franks changed this by creatuing a more or less cohesionate empire.
Maybe it should work like this: when a migrating people reached the top authoirty, becoming a cohesionate settled kingdom (Frankish kingdom under the Merovingians), it should enter the way that would lead it into either an empire (in the Byzantine sense) or a feudal monarchy. And this should only be done like the westernizing in EUIII, by a lot of factors, because the most likely would be to end like a Feudal monarchy. Which is not bad at all, Feudal countries were made from the war and to wage war. Feudalism was the key of Christian kingdoms in Spain to win the Reconquista against the civil society of the successive Muslim caliphates and kingdoms...
Some ideas I had while I rest of my studying. Take care!