Sounds interesting but I am not sure what you mean. How would you set that up? I am using only one era for the entire game.
You can then do it if you are ready to have "independant" tech tree.
Imagine you have 20 civilizations, and you want all of them to have their own city graphics.
You need to make 4 groups of 5.
Let say the group of Europeans, Asians, Middle East and Americans.
You make a tech tree for the Middle East group, with Era1 techs.
All these techs would require "Middle East group" tech, no era, and you give it only to the 5 civilizations of this group. So no one else will have it.
Then you "copy" this tech tree for Asians, except you set all the techs to Era2.
Then you "copy" this rech tree for Euroepans, , except you set all the techs to Era3.
Now, you edit the city graphics.
You have 4 eras, and 5 groups, so that's 20 different graphics. The only thing you do is to put the proper graphics at the proper place.
If you put France, Britain, Germany, Scandinavia and Russia in "European tech group", then you can say France is "culture group 1", Britain "culture group 2", etc.
The drawback is that France can exchange techs with Britain, but not with Egypt, as they won't be in the same "tech tree" and so not use the same tech tree.
Of course, you can play a bit with it, and change a few things. Like having some civs share the same graphics (ie same "tech tree" + flavour), to reach the 31 civilizations.
The important thing is that playing with culture group and era, you have 20 different city graphics availabe.