What is the most just social structure?

Applying Plato's definition, a just world is each performing his proper function. Defining human nature correctly and then finding a social structure which best allows the performance of real human nature is the task.

Human nature is a multi-level selection process between individual, kin and group selection, which helps explain the conflicted nature of human beings. Group selection sets the ethics in competition with other groups and their ethics, with the best bonded groups having the advantage over the less bonded people.

A world of thousands of small states or ethnostates, protected by a light federalism, seems to be the most just social structure because it allows each to perform his proper function in harmony with human nature. Perhaps Plato would agree.

If we then expand Plato's definition and say a just world is each performing his proper function with the sacred function defined as our evolution to Godhood, a world of many small states also harmonizes with this sacred religious function, since this sort of variety and, ideally, ordered-evolution, best enhances evolution.