I've been thinking of several different nations
---England, because that is where Monthy Phyton is based
---United Empire of America, based off some alternate history stories where the US spilt peacefully from the UK and formed its own Empire, with George Washigtion becoming the first King
---The Papal States, with a Catholic Pope at the head
---A fictional fedualistic state, The Empire of Ampersand
Assuming the mod will be basing this off England...
*King: The leader of the Empire. He has the power to approve or decline certain decrees.
*Heir Apperant: Appointed by the King. When the King dies, the Heir Apperant becomse the new King. (For the King to be elected by the nobles, it has to either be in Poland, Holy Roman Empire [Germany], or the Papal States, or some other kingdom...)
*Model Parliament: The Parliament system used in England. There were times when Parliament was more powerful than the King, and other times, notably the 15th Centruy, when the King was far more powerful than Parliament.
In 1265, Simon de Montfort, 6th Earl of Leicester summoned the first elected Parliament. The franchise in parliamentary elections for county constituencies was uniform throughout the country, extending to all those who owned the freehold of land to an annual rent of 40 shillings (Forty-shilling Freeholders). In the boroughs, the franchise varied across the country; individual boroughs had varying arrangements. This set the scene for the so-called "Model Parliament" of 1295 adopted by Edward I. By the reign of Edward II, Parliament had been separated into two Houses: one including the nobility and higher clergy, the other including the knights and burgesses, and no law could be made, nor any tax levied, without the consent of both Houses as well as of the Sovereign.
---House of Commons, One Half are appointed by Rural Counties (Knights of the Shire), the Other Half are appointed by the Towns. The House of Commons can only affect Taxation and Public Expenditures Laws. The House of Commons can be considered an 'elected' House, however, due to Voting Laws restricting the power of votes to those who have money, the electorate is conservative. If you want to turn the House of Commons Liberal, you're going to need to make voting laws more Liberal.
---House of Lords, The Nobles and Peers, herdieratry structure. "Advises" the King on certain issues. During the reign of Edward II (1307-1327), the House of Lords became incredibly powerful. It still kept lots of power until its power declined during the War of the Roses.
*
"Supreme Court"/"Lord of Appeals":
1) Either all Justices are appointed by the House of Lords, which had the judicial function as the "Court of Last Resort", or
2)
All members of the House of Lords can hear Court Cases.
Popular Revolutions: Occur if the King is unpopular or the tolerance of Political Violence is high. If the rebellion succeds, Change occurs. Otherwise, it's the Status Quo.
Glorious Revolution: If Parliament is opposed to the King, Parliament may introduce a motion to depose the King. If Parliament succeds, Parliament will then vote to appoint a new King to the throne.
Republic (unlikely to be added, but...): If society is liberal enough, Parliament may abolish the Monarchy and the House of Lords entirely and create the "Protectorate"/"Commonwealth of Nations". The House of Commons will be able to appoint a Lord Protector who will pass decrees in much the same way as the original Monarchy.