Here's a first list of Factions contributing various interractions, events, benefits or losses, imposing choices or unexpected forces at doors, and bringing some alea to the game :
TribalOrcs (Strongholds, good)
AlikrCoterie (SwindlerDen, good)
ThievesGuild (TheRift, neutral)
EastEmpireCompany (Solitude, neutral) -imperials/thalmor
BattlebornHouse (Whiterun, dark) -imperials
x
GrayManesHouse (Whiterun, good) -stromcloaks
BlackBriarsHouse (TheRift, dark) -thievesGuild/darkBrotherhood/imperials
x
SnowShodHouse (TheRift, good)
SilverBloodsHouse (Markath, dark) -stormcloaks
...more nord houses
BlackbloodMarauders (BrokenOarGrotto, dark)
...more bandits groups
DarkBrotherhood (Falkreath, dark)
x
PenitusOculatus (Solitude, good) -imperial
TheCompanions (Whiterun, neutral)
x
SilverHands (Strongholds, neutral)
VolkiharVampires (VolkiharCastle, dark)
PiquineVampires (MorthalSwamp, dark)
VulpinVampires (RedwaterDen, dark)
x
Dawnguard (FortDawnguard, good) -VigilantsOfStendarr
VigilantsOfStendarr (Strongholds, good) -Dawnguard
Some shall be added (some tribal reachmen, houses, bandits or rogue mage groups), some won't (like blades or greybeard), as the drake question shall be the dragonborn's. The dragonborn shall get in the list through.
While some events born from the rpg quests shall arise to the "main power" of a Hold at alea times, I'd like to give each major power a chance to influence once a turn a hold, bringing various consequences. With 2 dialog box for humanplayer (first to choose Hold, second to choose according actions considering many factors) and a part aleas for IA.