From Catherina II reign to 1871 hundred of thousands German emigrated to Russia. But the end of the open door, resulted in emigration almost stopping. So what if Alexander II decided to keep the immigration going? Even with the stop in OTL, the German population continue increasing through natural means, hitting two million in 1900, and only collapsed in the interbellum to 1 million and then double again after the War (through natural increase again). Of course only again to collapse again after the collapse of USSR.
Would we see a bigger Volga "Germany", would we see other German "homelands"?