[HoneyBadger]

This is a great idea, I love it. Considering it, I'd really like if there were a way to split up a map into several maps to create a Yggdrazil map where you've got a bunch of "worlds" around a central "world". I'm thinking the central one could be wraparound to simulate a round world, while the others could be fixed to simulate flat planes. Perhaps an additional "world" could represent the World-Tree itself, and be a long and thin "tower" with long, thin branches. You could then add special features to the map. For instance, in addition to the 8 Norse worlds, you could have an ocean world at the base of the Tree where the three ocean nations would live. This ocean would contain islands on which Sauromatia would live, and the central island would contain the cave (perhaps a cavern-system) of the Norns, who would be guarded by the immortal dragon Nidhogg-a buffed out hero-version of the Tarrasque. From the province of the Norns, you could go to any other province of the map-or atleast to every world, without having to brave the World-Tree, once you get past the dragon (he'd have to be beat every time you used the cave). The Bifrost province would be a Heat3 region that granted the possessor 1 gem of every type, and each province of the Tree would have some kind of nature bless or nature-based recruitable unit. Kailasa and Lanka might actually live on separate branches of the tree, since they're monkey-nations. Caelum could live in the top of the tree, while Pangaea would live at the base. Inside the Tree could be an extensive "cave" system of tunnels, within which could dwell giant blind worms (from which the Dwarf race were transformed). Vanir would live past Bifrost in the lands of Asgard, while Niefelheim would live outside these, past a mountain range with few passes, in Jotunheim, which would be the largest of the outer "worlds. The roots of the World-Tree would connect to three cavern-systems, the land of the Dockalfar where Yomi would be found, the land of the Dwarfs where Agartha would be, and the lands of Helheim.
Abysia would be found past the root system of the World-Tree and only connect to the Helheim lands.

The rest of the nations would live in Midgard.

Another possibility for the Norns could be the three ages of Ermor present on the map in separate provinces, each led by an independent Pretender-Lady of Love for early Ermor, Great Mother for middle, and Lich Queen for late era. They would surround the Nidhogg province, which would still be guarded by the dragon and connect to all worlds.