The Civilization series has taken players throughout history, but the next step for the series may be to discover new kinds of civilization underground. While the series' primary appeal has come from featuring various leaders and civilizations over the years, a more fantastical spin-off isn't out of the question. After all, Sid Meier's Alpha Centauri came out near the turn of the millennium, and it was essentially an offshoot of Civilization that starred a collection of fictional space colonies. While space may seem like a good expansion for the series, going underground could bring about more unique possibilities.