Okay, so after play testing the empire variant, we discovered that it doesn't work, primarily in that it gives you too many points early in the game allowing you to buy too many cards. Having pre-requisites helps a little, but only delays one turn. One of the things I dislike about civ is the end game, once you reach 9 cities, the game gets kind of boring, all the fun is in the first few turns while you are building your empire and expanding. So here's my idea, basically it requires extending civ an extra age, call it the post iron age. This means introducing some new 300 level cards. In a game I am running, we already modified the provincial empire card in that it gives you 10 extra tokens. My idea is that you it also gives you 2 more cities. You could then have an "Advanced Empire" (a 300 level card) which gives you ANOTHER 10 tokens and 2 cities. This allows you to exceed the 9 cities. for all cities over 9, you start over at level 2 and then proceed normally up the cards (i.e. 10 cities gives you another 2 card, 11 gives another 3 card, 12 gives you another 4 card etc).

Some other 300 level cards:

Advanced agriculture (science), adds another one to pop limit, aggravates flood (or drought if you use it).

Oligarchy (civic): Adds another 1 to your trade card limit, allows you to buy a second trade card of the same level at half price.

Divine Ruler (civic/religion): reduces city support by 1

Advanced Engineering (science/craft): same as engineering, cumulative with it, cities are impervious to attack if you don't have at least engineering.

This would mean a pretty long civ game (possibly even a year!) but would keep the game interesting, and allow players to drop out because you could absorb their units assuming you had the new cards.