We could do it Europa Universalis style where the map is divided up into provinces and the unclaimed provinces can be areas that are colonizable with nations who as sufficiently advanced enough and politically stable. We could also do a "United we stand" sort of thing for new players as their nations are a groups of provinces that have accumulate enough power and influnce to be recognized by the other nations or they could be a former colony of an older nation.

This also brings up some other points I want to make.
1) Is there only one land mass on the entire planet? Since the potential of oversees empires would be interesting. It would also give newer players a chance to jump in as it can be said that new lands were isolated from the current land mass as most nation were to focused on internal conflicts and issues than looking beyond the sea. (this is kind of also a selfish thing since one of the ideas my nation has is rapid expansion xD)

2) If we get sufficiently far into the RP, will gunpowder be making an appearance? Its only a matter of time before the more advanced nations create it and it will add a whole new dynamic to the game where nations who've had warrior class or trained, professional soldiers are suddenly outdone by recruits with gunpowder weapons. Of course, all nations will get unrest as the traditionalists wants to keep the old way of fighting in the face of "weapons of smoke and fire that kill without skill and without honor" and try to resist the change.