Abysia, obviously. Yes, they are the really shortest Fire-Giants so far, but they fit into the lowest level.
Niefelheim, Helheim, Abysia (Hey, this comes from Abyss, doesn't it?) and Agatha - I think the flair is really E-vil down there. Within this the contrast between Abysian "too hot" and Nieflheim's "too cold" - the lowest level is no place you will want to stay.
Ulm, Marveni, Ermor - the mid level belongs to the human, I thought Ulm to be a quite good central nation here, because - the Germanic and the Norse mythology are quite connected. - With all those forest survivalas, they make good guardians of the tree. The Other 2 were quite randomly choosen.
Vanheim, Caelum, Ctis, Tir Na n'Og - the high level should be something "above" metaphysically. Vanheim is easy. This map is about Heaven (Asgard = Vanheim) vs Hel(l). Tir took the nordic place of the Lightelves, I know they like that.
The other two ... I've choosen Ctis, because Pharaoes were supposed to be living Gods, and the Lizard Kings fit this role good. The Eagle Kings feel quite similar in this respect.