A slavic nation would be awesome.
But pretty please, no heavy emphasis on bears!..
That would be just too cliche. Bear-vodka-balalaika.... Hrrrrrrr.
Some random ideas for a slavic nation -
1) Early age paganism. Nature, Sun and Skies are worshipped, one of the major Gods of the pantheon (
http://en.wikipedia.org/wiki/Perun) being not unlike Odin of the Norse people. Magi (Volhv, pl Volhvy) - weakish astral mages and fortune tellers (bad events prevention) serving as priests. Spirits are seen everywhere. Military is represented by rather small, but well-trained retinues of the princes.
2) Middle age - contacts with Pythium are made, old faith is being replaced with a semi-christian one. Magi are being exterminated slowly. Composite bows are used widely, crossbows are known but not nearly as widespread, literacy is very common. Princedoms are *slowly* being united, cities are thriving, but...
3) Late age - nation is split by two, the western and the eastern. Cant tell much about the West, as for the easten one... Paganism is almost but-not-quite gone; same incursion that brought down the Tien Chi bureacracy nearly destroyed the country. Most major cities with libraries, cathedrals etc were burnt down and rebuilt. With the fall of Pythium (it fell just like Byzanthium, righ? A shame, if you ask me...) Church declared itself autonomous and heir to Pythium's and claimed considerable power. Archers with composite bows (crossbows are still known, but again, not widely used), medium to heavy infantry and medium cavalry compose a standing army. Constant incrusions both from the East and the West made people fiercely protective of their land. Conception of a 'bogatyr' arises - champion warrior, fighting for the people and the Church. I suggest giving these a prophet-like a dominion bonus, it represents the lore quite well.
And example of a LA national hero -
http://en.wikipedia.org/wiki/Alexander_Peresvet The painting isnt accurate though, according to the legend he wore only a monks vestment blessed by Saint Sergius since he was a man of cloth. ))
Saint Sergius himself -
http://en.wikipedia.org/wiki/Sergius_of_Radonezh
And, of course, the Three -
http://en.wikipedia.org/wiki/Bogatyr
The central one on the painting is also a canonized saint btw, officially up to this day.
Hope some of that may be helpful.
P.S.
To think of it... what I described as a LA should be actually MA in regard to the Dominions timeline because Constantinople fell later. But then in LA this nation should have firearms
So I'd keep it as is, Sergius and the rest of them are somewhere in-between the MA and LA anyway.