It does not seem to be the line of thought of the devs, they seem to maintain a rationale that some civs have to be included in all editions, especially those who have been in the game since Civ1 and who have already become "mascots" of the players, see Zulus being included in all editions while there are much better options in Africa, and look... Babylon is the only Civilization of Civ1 not included yet... I think this should not be considered, many civs are important enough to be in the base game and have not been there. Civs are included as new mechanisms are included, simply because some civs are designed for features that will only come in expansions. Sweden and Canada were included in GS to fit the World Congress and Maori because of environmentalism mechanics. Babylon may be being included later on for marketing reasons (a very popular civ left to boost sales of a third expansion, as well as Maya and Portugal), or perhaps Babylon is being designed for mechanisms that will come in 3xp, which can make it sufficiently distant from Sumeria. I think Babylon is the most likely civ to be included, not my first favorite, but leaving Babylon would be like, say, abandoning the Aztecs or the Mongols, I honestly do not see this happening.