Expanding the Northern Reaches: Jotunheim as 'Frostfell Mystara'
by Marc SaindonGaz7 briefly states about Jotunheim:
"]otunheim: This Outer Plane was granted to the giant races as a peace settlement concluding a long dispute between the giants and the lmmonals. It is inhabited by a variety of giant races, along with a number of other beings."
Kind of generic.
So why not make Jotunheim the area of norse culture on the Para-Elemental plane of Ice? "Frostfell" Mystara (Frost Giants might call their world "Ymir") would conveniently recycle some of the same maps as outlines, but here the temperature is much colder. Expect daily -20 celsius in summer and -40 as a 'warm day' in winter. Animals are the arctic versions of themselves, and often much, much larger.
Ylaruam is still a desert, but it's a tundra (strangely, it has camels as these were originally animals that lived in the ice age). The Frostfell Mystara region of Glantri might be home to the wizard frost giant Utgard-Loki (to keep things magicky), so this is a world that even Immortals find risky. Canolbarth forest is a more sinister Mirkwood or Ironwood, filled with troll-wives (hags) and fenrirs (dire wolves).
Frostfell Mystara has 'bleeding points' or overlap zones with Prime Mystara. These manifest when Mystara suffers a severe blizzard or storm. These crack in reality would typically be on already cold areas, like mountain tops. Giant creatures at times find themselves cast away into Mystara Prime, confused and trying to survive in a new world, which is basically seen as rampaging by local humans, and followed by a monster hunt.
The fabled Fimbulwinter of norse myth would be an event in which the barrier between Mystara prime and Frostfell Mystara is particularly thin for 3 years, until cracks start generalizing themselves all around, with both humans and Frost Giants seeing the other as the invaders in their world (letting the possibility of good characters on both sides to fix the problem before all out war).
Aside from the dominating Frost Giants, Jotunheim could have a large populace of Ice Genasi, called "Thurs" for norse flavor, and this might be playable option for PCs. They would be seen as enemies in the Northern Reaches, but be somewhat more adapted to explore Frostfell Mystara.
(also, I really don't like the giants' caste system in Forgotten Realms. Frost Giants and Fire Giants would do their own thing in their worlds, not establish a pecking order for the punctual moment they would ever meet).
other ideas might be found here:
and also in this 3.5 book, 'Frostburn', which covers winter adventures and cold realms.
(art from: https://edrogers.artstation.com/projects/8eR9KG)