There is also the unconfirmed possibility that culture will be allowed to spread, causing new recruiting possibilities, like in the Britain campaign of Kingdoms. It is only possible, since the exact details of religion/culture and how they will be used in EB2 have not AFAIK been released. Theoretically, a few of every faction's units could be available universally, and when a particular faction's culture becomes the majority in a province, then those units become available. Theoretically, that is, since that might not be as easy as I suggest, or the EB team might (probably will) go in another direction entirely.