A wonder I'm sure no one has suggested yet:
Masada. It's important enough to world history to be recognized by UNESCO; given the events and buildings in the Masada complex, I suggest a modest empire-wide boost to both Defense and Faith (say, +10%) and the appearance of a Great General. Of course, Masada would have to be built in a desert with mountains nearby; does Civ 5 have a way to include both restrictions?
Edited to add: A player-friend offsite suggested an additional restriction--requiring Walls in all cities before Masada is built.