In OTL Sweden was neutral and hesistant for French & British military aid to Finland despite being overall sympathetic to their eastern neighbor and sending them volunteers.
In this ATL Sweden not only has massive concern of Finland's ethnic Swedish population of Finland (which made up 10% of Finland's population during that time) what will happen to them under Soviet occupation, but is also worried that Sweden will be the next prey of the USSR if Finland completely falls, making Sweden not really a neutral bystander during WW2 in this ATL unlike OTL. While Finland is fighting for survival, Sweden comes with an offer for a personal union (Swedish-Finnish Union) where both countries have equal representation with dual roles for their governments: The individual is a hereditary monarch in one country (Sweden) and an elected president (Finland) in the other. The Swedish-Finnish Union is officially proclaimed in December 1939 of complete pragmatism and to lesser extend also Sweden's friendship with Finland to survive the Soviet bear. Just like OTL Finland during WW2, Sweden-Finland in this alternate timeline is neither an official Axis or Allied member, but simply a personal union country with the whole purpose of stopping Soviet expansion in the Nordic.
How would the Winter War and whole WW2 have changed if we pretend that the Swedish-Finnish Personal Union State had become reality with its main purpose being to survive against the USSR and Sweden giving up neutrality to help Finland survive?