If a Karelo-Finnish SSR was formed directly as a result of the winter war, Sweden might well have been forced into the Axis camp to protect it's iron mines. An Axis Sweden would mean that for much of 1941 to 1945, Finland would have been a battleground between the Germans, Swedes and Finnish rebels on one side with the USSR on the other. The war would likely have reduced nearly all Finnish cities and industrial centers to heaps of rubble and while total casualties for Finland might not be comparable to other parts of the Soviet Union due to much more lenient occupational policies (well, except for the Jews), they would be much more severe than OTL. In this TL, a postwar Karelo-Finnish SSR would probably recieve Northern Sweden down to Skellefteå (renamed Helettijoki if taken by the Karelo-Finnish SSR) with all it's mineral wealth to encourage the development of heavy industry in Finland and possibly parts of Northern Norway (if the Red Army occupied them) to compensate the Soviets taking Petsamo.
An Axis Sweden would also mean that Swedish speakers in Finland would be punished even more severely since they would be percieved as a security threat, with many of them liquidated or deported to Central Asia while the rest become Fennicized over the years as a result of policies introduced by the authorities of the Finnish SSR with support from the Soviet Union. Other significant demographic changes would also be the formation of a large Russian minority in major cities, with some cities closer to the border ITTL like Viipuri, Sortavala and Pitkäranta that would probably be 90% Russian like Narva OTL due to soviet population transfers, while other cities in the SSR like Petrozavodsk might even become almost completely de-russified due to Finns and Swedish speakers being relocated away from militarily sensitive areas.
The Karelian Isthmus would largely be annexed to the Leningrad Oblast like OTL, with the border ITTL probably being close to the Gulf of Viipuri, with Koivisto serving as the likeliest border town on the Finnish side. The Gulf of Finland Islands would be annexed into the USSR as in OTL. The Åland Islands would almost certainly be cleared from any inhabitants and turned into a massive naval complex like Kaliningrad OTL and would be leased to the Soviet Union for 50 years, although the Islands would still remain Finnish SSR territories. The Finnish SSR would also likely host several closed cities, especially in Lapland and East Karelia.