Disclosure: I'm one of the few Civanatics still playing, and posting about, Beyond Earth. I'm a fan of the Future Age, science-fiction-near-future parts of the Civ tech tree.
Having said that, another person here pointed out in response to one of my posts that adding a fourth era would turn a 600 turn game (standard speed) into an 800 turn game. Granted there are ways for each era/age to last fewer than 200 turns, but that's still a dramatic increase over past games in the franchise and slightly counter to Ed Beach's comments about making the game not last tens of hours.
Map Expansion: These are still early days, but we know that the map expands from Antiquity to Exploration. It wouldn't surprise me if the map expands again when going from Exploration to Modern. What would happen in a fourth era? No expansion? Add something like
@chaosprophet described, aquatic cities? Which I
absolutely love in BERT. Adding an orbital layer would be way too complex; one can't really put cities up there. Colonies/cities on the moon?
Switching Civs Again: I agree with several folks here, that it would be hard to create interesting post-modern Civs to be led by Augustus, Hatshepsut, Franklin, and Confucius. It's hard to "build something you believe in" in a science-fiction / near future era. Doing something like a European Union, an Organization of American States, or ANZAC doesn't inspire me.