It's been hard looking for a successor when I'm primarily a nonfiction reader, but by chance I've stumbled into one:
The Eastern Vikings - Baltic Megacampaign
The tale of Russia has always been one of
fascinating cultural history, and 'The Eastern Vikings' has certainly found an interesting way to rewrite it. From the Finnic Estonian chief of a single Baltic island, to the predominant power in eastern europe that replaced Rurik of Novgorod -- the story of Sulev Karasi and his descendants is an underdog's tale filled with epic raids, bloody revenge, ritual sacrifices, not to mention the strangest Germany (Tamilosaxons!?) and most recently... lots of angry mongols.
It's also planned as a megacampaign, and
@RedTemplar has finished CK2>EU4 megacampaigns before so he certainly has the tenacity. I anxiously await to see this version of Russia.