Like carve out a place between Russia, Belarus and Ukraine in the former Pale of Settlement. Establish a bunch of Yiddish language schools there. Have it become a center of Jewish culture. Sure, there are already Ukrainians, Russians and Belorussians there but there were already people in the far east too.
It would have been a massive dunk if after one of Ukranian Fascist revolts Stalin had just said "Jews Land Now."
The downside is if it happened before WW2 that would not have worked out well.