1. Purchasing Trade Cards
If using the Production Cities variant above, you may wish to consider allowing players to purchase Trade Cards from any stack rather than simply the Gold & Ivory stack. If this is the Case, the cost of the trade card is equal to twice the base value of the commodity in treasury tokens. However, do not allow the purchase of more than a single trade card in a turn.
2. Land travel from Asia to Europe
You may wish to allow tokens to move normally from Scythia to Kuban and vice versa without the use of ships, to reflect that these areas do, after all, connect just slightly off the board.
3. Dissemination of Technology
Not my idea, but a logical one. Each time the acquire civilization cards phase arrives, count how many of each technology card are already held by players before purchase starts. For each player that holds a given technology at the beginning of the phase a 5 point credit toward that technology is recieved by anyone who wishes to purchase that technology that turn. Like any other type of credit, technologies bought during that turn do not give credit towards further technologies bought in the same turn.
The original suggestion I saw was that the rule apply only to technologies costing 100 points or less. However, my friends and I could not understand the game logic behind this and found that applying it to all technologies was much simpler and did not disrupt play.
Again, this one is not my idea, but is a good one. Place an extra trade card, marked "Caravan", in the gold/ivory pile at the start of the game. This card is a "wild card" and can be used to add to the value of sets that do not benefit from mining. When it is drawn, it may be traded, but it must be used in the turn it is drawn, or it is lost.
Once used, the caravan card is returned to whichever pile contains the commodity it was used for. When it is eventually drawn again, it is used just as before.
5. Early Finish
Mostly just common sense; if any player manages to acquire all of the civilization advance cards, the game ends at the end of the turn in which he or she does so. This is particularily important in games that use the Trade cities variant, the Caravan variant, and/or the Dessemination variant; all of which have a tendency to increase the speed at which advances are purchased. Otherwise, although that player has obviously surpassed his or her opponents, he or she must simply wait for them to catch up, unable to advance him or herself, while still being generally unable to slow the progress of the other leading players.