You know how features can be made to shrink and grow at different rates, such as Ice and Jungle and Forest. Well, with a graphic change to, say, Ice you could do an Atlantean continent that tended to shrink and that would form the time limit for the game. Atlantis (for game purposes should be the expanded Cyprus version) has a tech advantage (it gets a Medieval start when others are Ancient) over the surrounding other civs, but its big green-ice continent is shrinking.