Here are several example from my Majestic Wilderlands that may inspire you for your own campaigns.
The Tharian Confederation
This started out as an alliance of clans of the Tharian Horselords. The clanheads, later their representives, came together as a senate to elect an Overlord to lead them in battle and adjudicate dispute between the clans. After the successful conquest of City-State and neighboring lands, they admitted some of the conquered territories as full members, their nobility allowed to sit on the senate and elect the Overlord.
In the century of its existance the Overlords have engaged in additional conquest, mostly at their own expense, that elevated the office from one of first among equals to a true ruler of the confederation. A bureaucracy has grown from what was the Overlord's household staff. Today the power of Overlord is only matched if all the member of the confederation unite. Also the whole of Tharian culture is fraying as the mores of customs of the horselords are not up to task to ruling many of the sophiscated cultures they conquered. Adoption of foreign cults and following charismatic leaders is the order of the day in the campaign's present.
The Viridstan Empire
The Viridistan Empire was founded by a race of humanoid demons who managed to escape the Abyss. There has been three Viridstan Empires. The Third Empire was founded when the number of Viridians were declining. They are noted for their perscuetion of all other religion subsituting the Imperial Cult which elevates the current Emperor to godhood. They believe greately in divide and conquer. The Empire is divided into provinces. Provinces are divided into districts. Each province and district has a Governor in charge of civil matters and a General in charge of military matters. Viridistan custom and law is very strict on the separation of military and civil authority. At the district level there are Magistrates and Sheriffs. There also a district Senate which makes local laws and control local taxation. The senators are appointed by the Emperor on advice of the Governors and Magistrates. There are several empire wide organization under the sponsorship and protection of the Emperor. The Imperial Cult is one of them as well as the College of Artificers (focused on making magic items).
The various Ghinorian successor states have the closest thing to traditional feudalism in my setting. The Ghinorians believe themselve to be the chosen people of Mitra, the Goddess of Honor and Justice. Their earliest governments were theocratic. While this no longer true of present day realms, The Church of Mitra is still highly important in Ghinorian culture. By tradition the highest Ghinorian title is the Prince. Their equivalent of King is reserved for the rulers of first state that managed to unite the Ghinor several thousand years ago. Since it fall, rulers of subsquent Ghinorian realms have decline to assume the title out of respect. During the Ghinorian Empire the head of state was known as the Imperial Prince and was considered the Prince of Princes.
Beneath the princes are dukes, counts, and barons. Like in Earth's history, Dukes and Counts were originally imperial offices that were transformed into higher noble offices after the collapse of the Ghinorian Empire. The titles of Prince and Baron were holdovers from the earliest days of Ghinorian cultures. The collapse of the Empire also caused the imperial economy to collaspe along with the widespread use of money. Land grants became the only means which the successor states could raise and maintain troops for their defense.
However despite the bleak economic times, serfdom never took hold. The Church of Mitra was firmly against anything that smelled of slavery or involuntary servitude. Instead sharecropping became the norm for feudal lords dealing with landless peasants. Manoralism did take hold as peasants needed to pool their mergre resources as well as the protection of the feudal lords. In the last two centuries before the campaign's present the money economy has re-estabilshed itself and many peasants have managed to purchase the strips of land they work. Also the various successor Princes are working to separate civil authority away from the feudal lords. This has led to violent clashes in some regions. Also the Ghinorians are divided over how catholic their faith in Mitra is. One side preaches that any who accepts Mitra as their god is a Ghinorian with all rights and oligations. The other side believes that only those who are born Ghinorians are the chosen of Mitra.
Like the Tharian Horselords the Elessarians are a clan based society. However while the Tharian clans are large groups of extended families, Elessarian clans are confined to immediate relatives. Elessarians have kings and nobles, however they serve mostly only as military officers. The source of Elessarian law are the Trehaens, a groups of scholars. Many Trehaens pursue the study of law and act as judges and mediators in disputes between clans and individuals. If need be they can issue a writ to the local nobles calling on them to enforce a judgment. Decisions o a individual Trehaen can be appeal to a regional circle. From there a final appeal can be made to the King's Circle of Trehaens. The king is an adminstrator and the supreme military commander. He has a lot of respect but little power except in times of war or emergencies. Elessarian culture is highly influenced by the Elves, and out of all the various Wilderlands cultures they are the most apt to cooperate and interact with the traditional demi-human races (elves, dwaves, halflings, etc).
To me the Old School Renaissance is not about playing a particular set of rules in a particular way, the dungeon crawl. It is about going back to the roots of our hobby and seeing what we could do differently. What avenues were not explored because of the commercial and personal interests of the game designers of the time.