[LDiCesare]

European mythologies ae quite well represented:
Celts (Irish - Tir na'n Og, Eriu, Welsh/Bretons - Man, Gauls - Marverni)
Norse (Jotunheim, Vanheim)
Latins (Rome - Ermor, Pythiumn)
Greeks (Pythium, Arcoscephale, Pangaea)
Slavs (Russia - Bogarus)
Generic Medieval Europe (Ulm, Marignon, Oceania)

Africans get a small share:
Egypt is hinted at with some animal headed pretenders and eater of the dead if I'm right, among others (C'tis).
Zulus (Machaka)

Near East gets several:
Sumerians (Agarthan names, C'tis)
Isrealis (Gath)
Many angels from this region.
Persia's Shedu are there too.

Far east:
Japan (Yomi)
China (T'ien Ch'i)

India (Lanka, Kailasa)

Americas
Aztecs (Mictlan)
Inuit? (LA Atlantis)

So I'd say North America, Arabia, Africa have a quite small share. I'm not sure about Indonedia, since I believe the Ramayana is quite big there. Vietnam, Khmer, Korea maybe? I'm not very knowledgeable about these, although I have some Korean myth book somewhere.
Pacific islands are also lacking but I don't think their folklore or myths could provide much in terms of armies and monsters or spells.

I believe a North American inspired nation could do something with totems and animal spirits for instance.