Because all most plausible DLCs were mentioned in this thread, I'd like to go more in detail.
Republic DLC - A few great families, comparing for titles within the republic, being able to decide about family matters by themselves (marriag etc.), trade posts, elections of the Doge/Granddoge by the nobility, plots to change the republic into a system ruled by one family (like Medici in Florence, Visconti in Milano). The republics should have unique trade mechanics.
HRE DLC - In my opinion a HRE DLC depends on a church DLC, because it were the internal reforms of the chruch in the mid 11th century leading to the conflict between Emperor and Pope. The Emperor should be crowned by the Pope, his basic titles should be the kingdoms of Germany, Italy and Burgundy. An AI Emperor shouldnt just allow the church to appoint the bishops, instead creating Antipopes etc. Another important part in the history of the HRE were the city bounds in northern Italy. These could be represented by republics, creating a faction to demand lesser taxes. In history, I want to remember the first moves of Frederick Barbarossa to Italy to reclaim the old rights of the Empire. This led to a 100 years struggle. The election system in the HRE works quite historically, I think. A Kurprince system, as it was established finally by Charles IV in 1356 isn't that necessary, I think.
Suggestions for HRE bookmarks:
- Canossa 1076/77 (with the banned Henry IV and Rudolf of Rheinfelden claiming the throne)
- The beginning of Frederick Barbarossas reign (conflict with Pope and cities in Italy)
- The throne war either 1198 or after the assassination of Philpp of Svabia in 1208
- The end of the Interregnum and the election of Rudolf of Habsburg (with a really small demesne of 1 county)
Monk Orders/Church DLC - I think, of all possible DLCs, a church DLC would be the most difficult. The church has no dynasty, so you had to rule a state, instead of a family. Therefore, the church would need a strong ideological life. A high religious authority could make it possible to excommunicate rulers because of their actions, while with a low authority, the church would depend on the local Roman nobility. I think, the monk orders and their thoughs could play an important role. Franciscans, Dominicans, Cistercians and the Cluny closter system and their thoughts could be able to do a reform of the church, comparing with each other. The heresies (Kartharians, Hussites could be filled with more life).
- Pagans/Mongols DLC - Difficult,too, because there are lots of different cultures among the Pagans. And they arent that popular in the historical memory than the Christians. Although, most Pagan countries were converted to Christianity at the beginning of the game (Hungary, nordish states). I dont really know, how to make a Pagans DLC..