Why would Stalin's USSR take Finland seriously ITTL when it did not do that IOTL? The biggest single problem of the initial Soviet campaign against Finland was that it was really not a campaign at all, but merely a belligerent parade march that was based on the idea that after the first Soviet artillery barrage, the poorly armed and barely trained "White bandit" Finns would shit themselves, flee in disorder and their defence would collapse in front of the massed ranks of the mighty, modern Red Army, after which the Soviet units would move to occupy Finland and install a pro-Soviet government in Helsinki.
Now, many level-headed Red Army officers knew that the Finnish Army would not be an easy opponent, especially in the dead of winter, fighting in eminently defensible terrain (and had in the interwar actually written studies that warned against doing exactly what the Red Army did in late 1939). Stalin, though, had either purged such officers or would not listen to them, deriding them as "cowards". So, the problem for the OP would be to explain what ITTL causes Stalin to treat Finland as a credible enemy, not a pushover. Given Stalin's personality and the fact that the Red Army officers were by 1939 terrified of him, it is IMO hard to go around this obstacle.