In order to allow us to play as existing historical characters, we need to consider the potential for abuse by the player in which having brought himself to ruin in one game, he can just retire and take over the historical character that leads the force threatening them and deliberately lead them to ruin before returning to his original perspective.
I propose as a solution a system by which all positions in the civilization are given a particular score according to their relative importance, written along the lines of 10/100/1000/10000. If the player rises to that position, then their score is increased to that level provided it is lower than the player's existing score. The score however is per site government or per civilization and allows you to assume the role of any historical character whose position is scored equal to or less than the players score WITH the relevant entity.
The score is refunded if the player retires the character alive, or the character dies in a manner unavoidable to the player like old age. If the player's character manages to get killed in a violent manner then the player loses the equivilant score as that character is worth. So a king is worth 10000 and the player's score with that civilization is 20000 and the player has the king throw himself off a cliff then the player can play as the next king of their civilization, since he would then have 10000 points. If he kills off that king however then he is back to the bottom of the heap, playing as an adventurer along the lines we have at present.
Fortress mode converts into adventure mode terms because the highest ranking position in your fortress turns into a score with your own site government/civilization government once you retire. That would mean that if you played all the way until you were the capital, the highest position would be able to play as the mayor *of* your site or you could play as the king of your civilization. Equally you could play as any of the nobles in your civilization but not the mayors of any other site.