Any nation, or group of nations, decide that they want to take a vacation from being a nation and make a 'contract' with humans where the human will take the place of a nation for x amount of time.

Bonus 1-For a nation the human that replaced them has grown to love to being the nation and refuses to turn back into a human.

Bonus 2-The nations 'family' reactions to the new person claiming to be Nation A.