Here are some ideas I was thinking about an eventual new game
First of all, I want to beg you pardon for my not perfect english;
1)About the gameplay: mantain both victory points and the possibility of not being stuck with one nation only, but changing the way how this can occur. I don't know if someone here knows a very old game called Medieval Lords: Soldier Kings of Europe. Well, in that game, you play the figure of a noble minister who advice the king or duke or whatelse and chose what's best for his country. The good thing was that, after a tot of years, you could choose to move in another place and so change nation. Point were made by doing succesful action or conquer; in GI 2 that could be traduced in fulfilling historical objective of the chosen country. You compete against other nobles by gaining victory points and, at the end of the game, a score table is made in order to see who is the winner. ( this gameplay could help to allow the new nations to be chosen without playing a proper scenario and to help the player to concentrate himself on only one nation).
2) Introduce internal politic as follow: in the Late Antiquity and the High Medieval era one of the most important things was the relationship between the aristocratic members (both laic and ecclesiastical and barbarian or roman; but expecially the episcopate) with the new royal court. So, for example, adding the possibility to negotiate with your nobles by promoting them as bishop in the region, granting them fiscal exemption for example, thus gaining the control of piece of land. (losing income for stability). Or the chance for emanating laws pro or equal or also against the roman citizen either gaining income or stability on what you choose.
Other things could be the type of government that you can choose with differences: for example the Byzantine Empire was civilized so it could have a permanent army and in danger times also a call to army(high cost more tactical less man); while a Longobard Kingdom prefer to relieve to his dukes on a army creation.(low cost less tactical more man). Etc...
Sorry again for my bad english,
Greets