[quote=SyrianHamster]
Obviously, with the departure of the original players, we need to work out how we do this. Do I revert the abandoned provinces first to NPC control, and then open it? Or do I offer new players to take over them? Do I give new players free choice over any region? What'd you guys think?
[/quote]

Do it this way, put the player controlled ones under leave of absence, if they pm that they quit or don't respond in 1 month, open their nation up. They would still be conquer able but the new players have to expect a rebellion when the original owner appears.