OzzyKP
Deity
Every 20th century civ and leader is taking a slot that could be filled by literally anything else. It also makes the final age considerably harder to quarantine if it comes with a full civ roster. So I'm going to go so far as to say that a fully developed fourth age on par with the previous three treated as a full expansion would ruin the game for me.
Maybe it will take slots away from vampires, zombies, and Hercules.