It's been really exiting to know about "the Old Gods". It's especially interesting to read about Rurik's invitation to Russia in the preview. This implies that Russia would be given some attention.
Considering how before 988 the overwhelming majority of Russia's population (and also Slavonic lands of Laba, Odra, Visla and Balkans) was practicing Slavonic paganism, it's crucial for the better immersion in game to give it proper representation. In fact Slavonic peoples was the biggest pagan group in Europe of that timeframe. Some Slavonic tribes of Russia, like Viatichi, being superficialy Christianized retained their paganism well into 14th century. The Slavonic peoples having Romuva as their religion in current CKII is of course ridiculous.
So, for Slavic paganism we don't have a single contemporary compiled source, like the Norse Eddas (which were compiled in Iceland in 13th century). Instead we have info scattered into many works of the missionaries and preachers. We have plenty of contemporary info in church chronicles of the Bulgarian churches, Wendish dioceses and Kievan Rus' church teachings against paganism. Significant amount of info was gathered through folklore, archeology and linguistics.
Major Slavonic Gods are:
Perun the Thunderer - The warrior God, was a chief of the later pantheon throughout all Slavonic lands.
Veles - The good of wealth and magic.
Mati Syra Zemlia (or Mokosh) - The Earth Mother.
Mara - The Death Goddess.
Jiva - Goddess of fertility.
Lada - Goddess of love.
Yarilo - One of the most venerated solar deities.
Svarog - God of fire, sometimes described as a smith god.
There are dozens more, and even more of the lesser gods, consult wiki for more general info http://en.wikipedia.org/wiki/Slavic_gods
Some vids:
[video=youtube;Dab5g0LtMwI]http://www.youtube.com/watch?v=Dab5g0LtMwI[/video]
[video=youtube;wxBuxbDWsgg]http://www.youtube.com/watch?v=wxBuxbDWsgg[/video]