First off, I have no modding experience and this would have to be modded by someone other than me, I do however have a vivid imagination.
One of the things I loved about Morrowind was the Great Houses. I felt that it gave Morrowind a good feel and made the player feel a part of a sect of society, not simply a member of a guild but it gave the player a family. Such a "guild" is missing in Oblivion. Oblivion resembles the Roman Empire/Middle Ages in its layout so I structured my mod idea in the same way.
First: A History
In the final years of the Second Era, Tiber Septim united all of Tamriel under his rule, thus beginning the Third Era. What has been forgotten in the histories are the contributions of members of once-proud kingdoms and families. These families fought alongside Tiber Septim as generals, quartermasters, messengers, etc. The once proud families have left their mark on Cyrodiil by the landmarks that bear their names. The Colovs (The Colovian Highlands), the Nibens (Nibenay Basin and Nibenay Valley), the Jeralls (Jerall Mountains), the Valuses (Valus Mountains), the Poppads (Lake Poppad), the Arriuses (Lake Arrius), the Corbolos (Corbolo River), the Canuluses (Lake Canulus), the Larsiuses (Larsius River), the Rumares (Lake Rumare), the Brenas (Brena River), and the Strids (Strid River).
The twelve great families once worked alongside each other, but after the uniting of Tamriel, rivalries began to form between the houses. Factions arose. The Colovs united the Brenas and the Strids under their name, the Jeralls united the Rumares, and Arriuses, the Nibens united the Larsius family and the Canuluses, while the Valuses united the Corbolo family and the Poppads. While at one time each of the twelve great houses were equal militarily, the mercantile genius of the four "great" houses allowed them to rise to the top.
The Colovs controlled the land from the Highlands all the way to the Abecean Sea. This area of land was perfect for growing wine and potions, and the Colovs made a fortune selling out of the cities found in their jurisdiction: Anvil, Kvatch, and Skingrad. The Jeralls controlled the Jerall Mountains down to Lake Rumare. The mountainous region was perfect for mining and the Jeralls became expert jewelers. The Jeralls sold out of Chorrol and Bruma, increasing their hold over the jewelry market in that sector. The Valuses controlled the Valus Mountains and the area of land from the Silverfish river up to the Blue Road. The Valuses mountains hold untold amount of wealth in the form of iron, mithril, silver, and other metals, making them natural heavy weapons and armor manufacturers, selling their wares out of Cheydinhal in monopoly. The Nibens controlled the area all along the Nibenay Valley and Basin. The area of land was best suited to cotton growth and shipping. The Nibens made their money selling light armors and clothing out of Bravil and Leyawiin. The shipping business from the Topal Bay to the Imperial city also belonged to them.
The differentiation of goods did not mean that these four families would get along, however. Over time, the houses found that they could enter the markets of their enemies and begin to make money as a competitor. Now that any house could sell any good, problems began to arise. Often times, the Nibens would send goods to Anvil, only to find out that their trader had not reached the ocean city. Speculation began to amount that the Colovs were either killing or bribing the Niben merchants. Niben family members, or sometimes their "lesser" allies, the Larsiuses or the Canuluses, began transporting the goods personally. When a young Larsius man, Vitellus Larsius, was found murdered and his goods taken, a war began between the Colovs and the Nibens. Not wanting to miss out on a possible stoppage in the market, the Jeralls and Valuses began competing with their goods in the troubles Niben and Colov markets. The intervention only served to push all four houses into open conflict with each other. No alliances were made, every family was for itself.
The families began hiring guards and adopting retainers into the family. The retainers were adopted from the foremost fighting family of the nearest non-Imperial province. The Colovs adopted Redguards from Hammerfell and Bosmer from Valenwood. The Nibens adopted Khajiits from Elsweyr and Argonians from Black Marsh. The Valuses adopted Dunmer from Morrowind while the Jeralls adopted Nords from Skyrim. The family fighting lasted for years, until the Emperor's Imperial Legionnaires confronted the families with imprisonment if the fighting did not stop. After the fighting ended, the families retreated back to their holdings and began rebuilding their mercantile empires. All the families were now well involved in distribution of all goods from wine to heavy armor, however, an agreement was made, the Pact of Vitellus Larsius, named in honor of the first victim of the Great House wars. The Pact stated that the Colovs would sell their goods in Anvil, Skingrad, and Kvatch; the Nibens would only sell in Leyawiin and Bravil; the Jeralls in Bruma and Chorrol; the Valuses in Cheydinhal; the Imperial City was deemed free market territory.
Now the old rivalries are back, the Valuses are dipping their hands into Niben and Jerall territory, the Colovs and invading Niben holdings and the Jeralls have expanded their market all the way east to Cheydinhal. Here is where the player appears.
The player would witness an altercation between two members of two opposing families somewhere in his travels in Cyrodiil. After witnessing the argument, the player would be given a quest topic. The quest would be to uncover what they were fighting about. After talking to either one of the arguers, the player would be told to seek out more information. The more information, would be a book of the History of the Great Houses, where after reading, the player would be given the location of the holdings of each house. After a visit to each holding, the player could then choose to join one of the houses (only after visiting with each house).
The "holdings" would be castles/mansions situated in the respective countryside under that houses' jurisdiction. So, the Colov mansion would be in the Colovian highlands. Each house faction, would be comprised of a couple different members. Each "great" house also has two "lesser" houses to do its bidding. These "lesser" houses would hold small estates in the countryside. Each "great" house's mansion would have a pater familias (head of the house), mater familias (matriarch), a few sons/daughters living at the mansion, the household guards, retainers who conduct the business of the house, "lesser" house representatives living in the "great" house mansion, and servants. Each house would have sons/daughters living in the city under their jurisdiction, conducting business affairs in that city. There would also need to be a "black sheep" of the family working with a rival house, or simply squandering the family's money on frivolities.
The "lesser" houses would have a small estate populated by the patriarch and/or matriarch of that family, perhaps some sons or daughters, and servants. "Lesser" houses would have no retainers.
After joining the family, it would be the player's job to perform tasks for the family patriarch.
1. Consolidation of the family would be the first goal. Reuniting the immediate family (i.e. tracking down the "black sheep").
2. Following this a push into the market economy. Having to find sellers for the family goods in the closest city.
3. Then a re-establishing of the "great" house could happen. Perhaps the "lesser" families are trying to separate themselves from the "great" house. It would be the players job to either outsell them or somehow put that family back in its place.
4. After the Houses hold on its original jurisdiction had been completed, the family could look to further their interests. A push into the opposition could be a quest.
5-8. As well as both creating and canceling alliances with the leaders of the other great houses, throughout the questline.
9-10. After establishing the player's house as the most powerful merchant family in Cyrodiil, the other families may feel a military reprisal is in order. Leading the guards and retainers against rival houses could be a quest.
11. Or buying out a family and making them a "lesser" family in service to the player's family could be a quest.
12. In the end, the player's house would be the only one left. Once this is the case, the player could become head of household by the support of the "lesser" houses, the retainers, or by his own accord. Or, the player could be named Heir of the "great" house to avoid bloodshed.
Getting to this position would men great monetary reward for the player, as his House is the only house left producing goods for Cyrodiil. A weekly profit could appear somewhere in the Great House's Mansion. Perhaps the player would be offered a seat on the Imperial Council. Free goods comprised of the House's specialty could become available to the player.
There could be about 12 quests, or more. These would provide about 6-7 family ranks.
Ranks - without Oblivion style names
2nd in Command (Underboss)
Advisor (Family Counselor)
Sorry for such a long post. But this is something I thought about a year ago and wrote it down. Just found what I wrote down and I felt like sharing.