I think its far more likely that Russia could just turn out to be something non-slavic (or a different type of slav) than it being conquered. Sweden, Poland-Lithuania, Lithuania, and Uber wanked Germany could all possibly pull it off (with Germany being the least likely), but in my mind, the most likely invasion from the west is something like this:
Without a world War I, Russia has a communist revolution in 1923. A host of international powers including Germany, Poland (maybe if it got its independence in time), France, Britain, Japan, and possibly also the U.S. and China invade and balkanize Russia.
Also, I highly doubt that a unified power could ever emerge in Europe for long. The mountains and rivers are a lot more foreboding than those in China, east of the Tibetan plateau, and hence form barriers that not only foster individual nations, but whole ethnicities and cultures.