Hence the focus on the Kievan Rus to keep it simple enough ingame. But yeah some flexibility is required for a game that covers over 1000 years.
As new slavic civs have been added (Bulgarians Poles Bohemians), that’s why I think Slavs should be renamed ingame. And the medieval term “Ruthenians” looks like the best choice.
Kievan Russians were Vikings:
The Varangians (/vəˈrændʒiənz/; Old Norse: Væringjar; Medieval Greek: Βάραγγοι, Várangoi; Old East Slavic: варяже, varyazhe or варязи, varyazi) were Viking conquerors, traders and settlers, mostly from present-day Sweden.[4][5][6] The Varangians settled in the territories of modern-day Belarus, Russia and Ukraine, and in the 9th century, they founded the medieval state of Kievan Rus’. They also formed the Byzantine Varangian Guard, which later also included Anglo-Saxons.[7]The Varangians settled in the territories of modern-day Belarus, Russia and Ukraine, and in the 9th century, they founded the [medieval] state of [Kievan Rus’]. They also formed the [Byzantine] [Varangian Guard], which later also included [Anglo-Saxons]
At first yes, like the Normans (who assimilated into the Franks and would become both the Britons and Sicilians ingame). Over time a new culture appeared, the Vikings adopted local languages and religion (well, the Rus converted to orthodox christianity under the influence of the byzantine emperor Basil II, I don’t know if before that they had converted to slavic paganism). Making them an orthodox russian-speaking civ.
If I had to make a campaign about the Kievan Rus, I’d indeed start as the Vikings, then switch to the Slavs after the transformation is notable enough (converting to orthodoxy is a major event).
You gotta look at the timeline:
First, Kiev Rus (vikings) - about 900BC
Then it breaks up, northern part (Muscowy, Novgorod)1100-1400BC remains their, while the southern parts(relative Ukraine) begins changing hands between Cumans - Mongols - Tatars - Turks etc. etc. untill the timeline of the game ends.
I think the slavs were intended to represent the Novgorod/Muscowy regions. The fact they have Alexander Nevsky hero in the game, who lived in the 13 century backs up this claim.
But Novgorodians/Muscowites is kinda lame, if you named them just Russians, then you will encompass all those smaller states, while a bunch of people would indentify with them (sadly the current situation doesnt’h help though).
Calling them “Ruthenians” would cover the same, minus the troubles about current events.
“Byzantines” term is a historiographical, there weren’t other nations that called them like that while they existed.
Regarding the topic, “Byzantines” term is not related to anything that existed after the fall of the Byzantine Empire. Unlike “Ruthenians” term.
Kievan Rus’ and its people, refer precisely to the era and the state that existed between 882 and 1240. That’s in-game. If that’s not, then I pass.
“Ruthenians” refers to late medieval and early modern east slavic population of Lithuania and Poland. This is confusing if the in-game civ is the Kievan Rus’ specifically.
Aoe2 decisions based on current events? That would be a crime against aoe2 and reason 
…
The term “Ruthenia” was used in the Middle Ages to mean the Kievan Rus and the Kingdom of Galicia-Volhynia. It indeed also meant the parts taken by the PLC in the early modern era, but for the medieval era it would be correct
The name “Ruthenian” was also used to mean “Eastern Slavs” in the Middle Ages, only later did it tighten to mainly Ukraine and Belarus
So for the medieval era, it would be correct. Especially as the civ has the symbol of the Kievan Rus. For the same reason, it’s well understood that the Britons are the English after the norman conquests, and not the Romano-Britons (Romans or Celts) or the Anglo-Saxons (Goths and Vikings would be used as placeholders).
Just keep it simple. Rus.
Rename them to Cheeki Breekians.
Make a next poll which name fits best for slavs rus russian ruthnian varangian other.
Yes, but so was Kiev after Mongols arrive… just call them Ruthenians and call it a day.
The problem of Russians and Ukranians have nothing to do in a game
That is not a historical fact and heavily disputed, mostly for genealogical, cultural and ethno-linguistic reasons. There is a strong difference between the cultural imprint the vikings had eg. in britain, compared to russia (and lack thereof). In fact, baldly claiming that the rus were basically a german colony is following track of such prominent “slavophiles” as dr göbbels. The fact remains that varangians were of different ethnicities, and most likely they were slavs and baltics from the southern-western shores of the baltic (at that time, the varangian, not viking) sea. See obodrites as an example of such now vanished tribes.
Well, this is the official AOE2 description of the Slavs civ:
While new kingdoms formed from the ruins of the Roman Empire in Western Europe, Slavic tribes settled the rich lands of Central and Eastern Europe and established formidable states of their own. Illuminate your populations with the teachings of Orthodoxy, recreate the rich farming and trade economies of Poland and Kievan Rus, assemble retinues of Boyars and Druzhina to defend the Russian principalities from the Mongols, or lead a charge of Winged Hussars to save Europe from Ottoman conquest!
—Description
And the aoe wiki tells us that they represent the East Slavs:
East Slavs Wikipedia information tells us that, only in a part of the Rus’ people (southwestern and western, subject to Lithuanian and later Polish influence) the Ruthenian identity was developed.
So, Rus’ is the collective term, or simply East Slavs.
How old is this description ? It seems outdated due to the introduction of new slavic civs as it clearly mentions the Winged Hussars, who are now clearly Polish (and Lithuanian but they are not slavic).
As old as “The Forgotten” expansion.
Indeed it needs an update.
There’s four slavic civs including the one called Slavs. That’d be like if Hindustanis stayed as Indians despite DOI.
What about the Italians?
They’re weird, that’s what