Athens and Sparta together, when they were sworn enemies more often than they've been allies.
What's the problem with India or China? They represent their modern states and then they go back in time. Unlike Greece, which represents only it's Ancient culture.
Uh... you're kidding right?
Both India and China represent far more than just the modern states (particularly as China has a 7th/8th century ruler). The point of the game isn't that it represents a bunch of small independant political entities, but instead Civilizations and Cultures. That's why we have Civilizations like Germany, England, Greece, Polynesia, Mongolia, China, India etc. rather than Prussia, Bavaria, German Empire, United Kingdom, Athena, Makedonia, Sparta, Hawaii, Aotearoa, Yuan Dynasty, Golden Horde, Ilkhanate, Chagatai Khanate, Zhou Dynasty, Ming Dynasty, Qing Dynasty, Mughal Empire, Maurya Empire... and so on. If that's what you really want, I would suggest looking into Paradox Games' products (they are quite frankly fantastic if that is what you're looking for), but that is not what Civilization is or about. The Civs in game are always going to be more general, as they are Civilizations, not Nations, not Nation-states, not short lived political entities. They follow the people and culture throughout their history, and whilst some have relation to eachother, they always have their own identity and culture.