I think the Six great nations would be the ones to form the union of allied countries against Nazarick.
Most of these countries are probably stronger than all the human nations shown so far apart from the Slane Theocracy. I believe three (3) of these countries ate humans as a food source like the Beastman country that is attacking the dragon kingdom.
We still don't know if some of those nations also refer to human nations. The Slane For example could be one of these Six Great Nations. The Only One we know off is the Troll Kingdom.
What do you think?