With a POD after 1900, have St. Petersburg for at least a year during (relative peace) become part of a Nation which is neither Russian nor Nazi (nor Nazi clone).
I don't see a victorious WWI Germany taking it away from the Russians unless the Russians *completely* go to pieces (a *worse* Russian Civil War?) and probably not even then. (They might support a German aligned government in St. Petersburg, but making it a full part of the empire?).
Can the Finns take it (would they want it?), Estonia and the other Baltic States seem unlikely.
The other options would seem to be a post WWI Poland Wank or *maybe* a late 1940s WWIII.
Any other ideas?