What's behind the Presidents Head?

Behind the president's heads is a small valley.  During construction many buddings were housed there and on top of the heads.   Including an office, bunkhouse and workshop to fix the drills.  Over the years politicians have campaigned for different people and presidents to be added.  I'm sure one from their own parties.  Thank goodness there is not enough room.  But who else could you add? John Adams, Martin Luther King or maybe P-Diddy?