The subject has been tackled quite a few times already and is rather popular. I also played your Finland 1918 which is great, so the "second round" war Finns vs Soviets could be nice too, and a challenge to create, due to the disproportion of forces and means.
I'll be working some general ideas soon, but here are my first hand ideas from what I understand of WAW system.
1 - Soviets forces (divisions and brigades) are numerous and strong (in numbers), but weak in moral. Strong airforce, also in numbers, but dense forest terrain will render it not that powerful
2 - Soviet leadership is abysmal, leaders will be mostly 0 in values
3 - Finns will have small units, low in hit points, but very high moral. Quality of commanders will be high too. So overall they have the edge in most battles, if they survive the attrition
4 - Cards will allows to render some decisive aspects (ambushes, winter war fighting, various supports, Mannerheim line, Stalin insistance, international diplomacy and politics, etc..)
5 - Finns can win by holding (up to a point) or by provoking the Allies' intervention (so a tension index system will be used), Soviet wins by capturing the border regions they demand and forcing Finland's capitulation and/or favorable peace.
Some other things I should include ?