The plan for expansion is twofold; I'm following a general master expansion plan, but I give my vassals complete leeway in choosing a suitable location to settle. In other words, I am roughly proceeding from this expansion plan for three duchies (of which some of these are already in development) which aims to maximize the variety of the lands incorporated into Wysteria:
That being said, with the vast amounts of free land around I don't feel much requirement to hold strictly to this; so if you were to want to settle, say, some of the regular or cold taiga just to the south of the ice plains and in the bottom right corner, I would have no problem with you doing so. In all, you would be free to settle just about anywhere you wanted.