My ideas so far:
HYPERBOREA
1) Geography - Plane. This is set on an alternate demi-plane somewhat linked to the real Earth's north pole. It reflects a distant past, before an unknown northern continent (the one broken into four parts in the center of the map) sunk under the ocean, and before ice covered all of the north pole. Note the place is not covered in ice and freeze, rather being cold-temperate. On the other hand, you have nights and days as near the north pole on the prime plane.
2) History.
-- One should know that more than 12,000 years ago, a layer of ice miles thick covered most of the northern hemisphere. Then it melted, making the oceans' level rise up to 400 feet high. However, farther back in time there was almost no ice, but this continent (the one broken into four parts in the center of the map).
-- In the distant past, this continent was home to warring alien "gods" (of the Cthulhu like kind; think Tsattoghua, Itaqua, etc.). The "god" related to ice and cold won, and during earth shattering events the continent sunk under the waves (the origin of the Atlantis myth?). Thereafter the ice did spread and covered most of the northern hemisphere. However, at some point the ice melted and retreated, only remaining on the north pole for 12,000 years. It's because the "god" of cold went into slumber or maybe was fought back by the others.
-- In any case, during the mighty magical war that saw the victory of the "god" of cold, some kind of alternate plane was created, where the vanquished "gods" took refuge. Now, events in this place are affecting the real north pole (on prime material plane). The war is again fought, and its outcome will affect the normal world. Death of the "god" of cold will have the ice defnitely melt and the ocean rise much; on the contray his victory will bring back the ice age....
3) Magic.
-- Note that the mountain at the center of the map, on the north pole, was a powerful nexus of magic, to which ley lines connect. As a result, on normal Earth (outside of this alternate plane) the world is low-magic at best (and near the ley lines). When on this alternate plane, magic becomes easier, and more important near that mountain. There is more magic and magical beasts on the central continent than on those on the border of the map.
4) Mortals in this place. Several ideas (which one do you prefer?):
-- a) This is a typical Call of Cthulhu adventure, set in 1930 or in modern times.
-- b) This a Robert E. Howard world set in the dawn of times, with black magicians and barbarians.
-- c) This is a weird place where humans from different epochs ended (unable to get back on the prime material plane), and continued their existence and older civilizations. Hence, there you find Vikings, but also Celts, medieval-ages' people (with priests and knights in plate armor) in their medieval cities and castles, etc. All of them don't know what happened in the real world (and have probably forgotten about it).
-- d) Of course you could also go with a+b+c...
5) Adventures.
-- Figuring out what's happening here, what this place is.
-- Eventually becoming involved in the war between the old "gods" that is being fought again.