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With a POD of 1900, do so that in 2017 there are more East Slavic nation-states than just Belarus, Russia and Ukraine.
The only plausible one I can think of personally is a Rusyn state. Other alternatives could be a revived Novgorod, although that would require 'fabricating' a new culture, but it's arguably the most famous extinct East Slavic country (Ukraine is often refered to as the successor state of the Kiev Rus, if I'm not mistaken).
Perhaps some radical east Slavs decide to set up a Iazychie nation-state, in the form of an anarchist or communist state?