I just hope you fix this ridiculous internal migration. In 1836 Mazowieckie Region has 600k pops, by 1880s it had 160k in my last game. Warsaw alone having 10 times less than in beginning. Now tell me why, WHY, would Polish and other people move from one of the most fertile lands in Russia and one of most human/industry/farming friendly climates, to say Siberia where Farming and pretty much industry is simply impossible due to ridiculously cold temperatures? Even moving further east would be quite out of question for most of those people, since there were strong Nationalistic Movements at that time and unless forced to do so, most Polish people wouldn't move to native Russia areas.
It seriously let me down, I tried playing campaign with Krakow, I won war badly outnumbered hoping I'll embrace 500-600k of native pops, only to find out the region is completely empty..Forget about assimilation, there was nothing left to be assimilated to Russian by that time.
ps. Another factor is rapid urbanization around the time period in which game is happening, it can be easily seen if you lunch 1861 campaign. Berlin in 1836 start has 200k pops, by 1861 it has 1.1kk. What happens in game, is instead total depopulation and opposite of urbanization. People for some bloody reason prefer to live on Sahara, Siberia or in Jungle far from civilization, newest healthcare and technology, which could be found in biggest cities(ie capital provinces of majority regions). Why it happens is beyond me, but it'd be total game changer to see some change that would enforce those trends, as after all this game is mostly about industrial revolution.