There will be two categories of Nations in Vicotria 3, that every nation can be classified by. These are the Centralized and Decentralized nations, that is the colonizers and the colonized.
Centralized nations are, in the most basic terms possible, nations with a central government capable of enforcing rules and regulations over all of its citizens and territory.
On the other hand, Decentralized nations are a step-down from Centralized ones, at least when it comes to governance. They have no officially established central government and have no way of establishing and enforcing nationwide changes. In other terms, Decentralized Nations are those whose "governments" are not a singular entity as the western world would know it.
Paradox developers are very forthcoming with information about all of their projects and interact with fan communities quite often, and it was no different in their latest diary. They have confirmed that Decentralized nations will not be able to become centralized on their own, at the game's release, in answering a question posted on the forums.
This would make historical sense, as Centralization as we know it is a man-made concept and not a natural state of being. It would not make sense, historically and socially, for the tribes of Native North Americans, for example, to all of a sudden adopt the governmental polities of Europe, without coming into contact with them.
And knowing what the practices of Europe were at the time were, there is no way for these two entities to come into contact with one another and not have the technologically advanced Europe try and colonize the native land.
However, Paradox have made sure, by mapping out the native territories, to give the native populations as much of a fighting chance as they could, in the case of a native uprising.