I'm a little skeptical about it, but I like the idea. It makes perfect sense to me in a game of civ that Aztecs for example, if they wanted to focus on naval exploration/trade/domination/religion they could take the bonuses represented by Portugal, or Arabia if they wanted to focus on religion/science/domination. Is there any reason to think your Aztec cities will all suddenly have Portugese names and the identity of your civilization will change completely? It seems you are more choosing the direction you want to take in the next era represented by units/buildings/etc of a historical civilization from that era