@Nathan-Greve
How interesting. We haven’t had this be an issue in games yet, but I hadn’t considered that Iceland, for example, is just totally unusable now if it’s really all Tundra! A shame for Iceland specifically, as I believe for a time it was used as a weigh station for convoys heading to England.
Agreed with @Trig on some of the expansions, I don’t think they were really modified completely (properly?) to integrate seamlessly to V3.
But speaking of expansions, one fun house rule you could use for Tundra might involve elements of the Winter War expansion. Namely, use Ski Troops in your game. Maybe only Ski Troops can access Tundra territories/borders or something to that affect? That way there’s still a restriction, but you can have a work around by having to by specialized units to move/combat within the Tundra zones!