Plato's "Republic" comes to mind.
Edit: as Yannis suggested, I've got two reasons why the Republic qualifies. A part of it discusses how to organize the most effective state, in what classes should its population divided into, how to organize educational system, who should take care of the kids, how to properly brainwash the populace into obedience, etc. This discussion is not tied to Athens or any other specific city-state, it's more abstract than that, therefore one can say that the proposed design applies to the Humanity as a whole.
Much later edit, in response to @LennartRegebro comments. Below is a portion of one of the dialogs from Republic that aims at justification of distinct classes in the society. Notice that the end purpose discussed how to "make the city just", not how to benefit one class or another.
"Think, now, and say whether you agree with me or not. Suppose a carpenter
to be doing the business of a cobbler, or a cobbler of a carpenter;
and suppose them to exchange their implements or their duties, or
the same person to be doing the work of both, or whatever be the change;
do you think that any great harm would result to the State?"
"But when the cobbler or any other man whom nature designed to be a
trader, having his heart lifted up by wealth or strength or the number
of his followers, or any like advantage, attempts to force his way
into the class of warriors, or a warrior into that of legislators
and guardians, for which he is unfitted, and either to take the implements
or the duties of the other; or when one man is trader, legislator,
and warrior all in one, then I think you will agree with me in saying
that this interchange and this meddling of one with another is the
ruin of the State."
"Seeing then, I said, that there are three distinct classes, any meddling
of one with another, or the change of one into another, is the greatest
harm to the State, and may be most justly termed evil-doing?"
"And the greatest degree of evil-doing to one's own city would be termed
by you injustice?"
"This then is injustice; and on the other hand when the trader, the
auxiliary, and the guardian each do their own business, that is justice,
and will make the city just."