A main obstacle to having adventurers as position holders in sites or civilizations is that position holders do not have deputies to take over if they are aboard. Adding in deputies, appointed either by the position holder or by the site to take over from leaders before their departure who are unable to presently carry out their position duties would solve that. This would make it possible for us to make all historical characters playable as adventurers in adventure mode.
If we decide to play for instance the king of a dwarf civilization we get the opportunity to assign another character to be the regent, if we choose not do so the regent is assigned automatically by the site. As a place-holder mechanic, being an adventurer is considered to automatically disqualify the king from his duties, so we do not have to explicitly code the king actually going about being king but that option is there for future development.
The regent or deputy continues to function exactly as the position-holder they replaced in every respect, until the real position holder returns to the site (or capital) and retires there. The same system also has potential in fortress mode, not only would it facilitate our position-holders going off eventually to political meetings but it would also facilitate maternity for the likes of squad leaders, captains of the guard and militia captains.
When one of those leaders gets pregnant or has a baby, we would be able to appoint a deputy from within their squad to replace their role until their babies have grown into children. This way we avoid having our dwarves spam the fortress with engravings of said military leader getting dismissed and then reappointed. The same thing applies for when the leader (or any other position) is temporarily rendered unable to perform their duties as a result of injury, but we do not wish to actually dismiss them from the position.