Looking at the results of the poll, I wonder what ya'll don't like about this idea.
Why would it not be fun to plug another era into the game and play the whole game with it.
The scenarios in Civ 2 were rather limited - and you could thus only focus on certain eras instead of a broad range.
Also, you always know what the scenario would be like - it's the same every time.
Looking at the tech tree for Civ 3 it seems they may even have the game divided into eras if only somewhat.
See here:
http://www.gamespy.com/asp/image.asp...iv3/image4.jpg
Personally, I would love to be able to drop in an alien era at the end (and add a few more years to game) so I can have a good challenge at the end fighting aliens or something.