Traditions, religions, aesthetics, culture will be free for the player to settle(as long as it won't contradict with the strengths and flaws of a nation.). As for a nation's background, other than a few key moments in history, such as the Demonic Invasion and their conquest by the First Celestial Empire, the Second Celestial Empire and the Third Celestial, you are free to make their background as you wish as long as it doesn't conflict with the backgrounds of other players or the strengths and weaknesses of their chosen Kingdom/Empire, even during their times as autonomous territories under the empire.

If you wish to know anything about the the events I have listed here, just feel free to PM me.