I'd been pondering some ideas recently that I wanted to make into a thread. Maybe here is a good place to bounce them off people because it's apparently relevant and it could help the discussion.
I was thinking about revolutions and rebellions. Perhaps settlements, big or small, could experience a peasant revolt in which they overthrow the local aristocracy who govern them. Their leaders, nominated by the people or perhaps themselves (think of demons and vampires taking over civs), then become rulers of their own peasant society. Depending on the personality of the rebellion leader(s), they could appoint themselves as "king" or "ruler" or some such title, perhaps as inoccuous as "leader".
A more selfish or conservative rebel leader may go for the traditional royalty, whereas a more compassionate jughead would maybe give himself a title like "Father of the People" or something and rule a kind of proto-anarchist/communist territory of his own. Those who rebelled for religious reasons could nominate a religious leader as their civ leader, "Great Poobah" or some such, again depending on the personality of the main rebel leader - whether or not he is particularly evangelical etc. Theocratic settlements/civs like that could impose strange and bizarre laws according to their religion - insisting that all Dwarves have moustaches not beards, children stay indoors and everyone must pay homage to the earthy monkey god, for instance. Think Taliban. Republics and primitive Greek-style democracies would be possible to an extent.
Rebellions would take place among the peasantry generally when the peasants become dissatisfied with a particularly weak or cruel ruler who has made their life hell, or when the religion/race of the populace differs hugely from the race/religion of the rulers. An example would be a human civ following one god conquering an elf territory whose populace generally supports a force. They rebel on the grounds of racial and religious oppression/assimilation/discrimination. Another would be a demon wandering into a village and converting most of the populace to his cult. The cultists lead a rebellion against the local leaders and let the demon rule the town, leading to all sorts of craziness (think of whole villages filled with zombies, black magic and horrible stuff that you have to cleanse. I mean, walk into a local tavern and see the drunkard being crucified upside down and everyone has red eyes).
If a civilisation loses a province and it rebels, depending on how the civilisation treated that province (and depending on the loyalty of the leaders and the province's wealth), then it may rejoin your civilisation. If the province was particularly rich, they may try to preserve their independence, meaning you will have to strike them down. If a civilisation is conquered completely and one of the provinces of that civ disagrees, there is a chance (again, personality of the leaders) that the rebellion that takes place may actually nominate itself to represent the old civ - their leader could, as an example, appoint himself Protector of the lost Dwarven Kingdom of Purplecloak and rebel against their goblin oppressors. When they regain territory and unite with other rebels, they would eventually reform their lost kingdom.
However, the legitimacy of these "civilisations" would be very questionable, and would probably get laughed at by local kingdoms and conquered completely (fair game). The legitimacy would increase depending on the status and rank of the rebel leaders, the highest being a particularly famous and popular Duke, or perhaps a widely respected King who has lost his old kingdom and decided to found a brand new one. If the leader was a count, he would be less likely to be accepted by other kingdoms and would be more likely to experience the agony of being "fair game". Same goes for baron, mayor, expedition leader etc. Other kingdoms who are enemies with the home civ of a rebelling province/settlement may be more likely to recognise you and support you to cause problems for the home civ. This would only be likely if you had the right credentials though i.e. you should be the affluent, newly-declared Kingdom of Regalhams, not the dirt-poor People's Free Territory of Tumblescrubs.
Here's the point - your fortress would work in the same way, right down to the peasant revolts and gaining recognition. You could declare independence from your fortress and rebel against your rulers with just 7 Dwarves. It would be suicide, but you could do it. You could declare independence with a Duke and 300 merry men, fight a war (possibly whilst gaining support from enemy civs), secure your independence then rise up again and conquer your home civ. Anything would be possible, and you would be able to choose what shape your civilisation would develop into. It may be that you will end up with a Dwarven Republic or even a Free Territory that rules an entire continent with local democracy and all sorts. Who knows, it's up to you. Ultimately central to all of these things are well developed and complex leadership and the nobility mechanics - without them, none of this is possible.
Perhaps the general makeup of civs could change over time. At first (age of myth etc), basically everything is a Kingdom. However, as time goes on and rebellions are successful, you may see by year 500 some large Empires and Republics appearing. This could affect the recognition of rebellions - a Republic may be more likely to recognise the legitimacy of a Republican rebellion and so on. It's not so out of place considering the time period - Republics and the like have existed for a very, very long time, at least during the period DF is concerned with. Remember, it would all be very primitive though - so voting in a democratic settlement may involve the local men (or women, depending on the society) getting together in a town hall and saying "I agree" or "I disagree".