I think tourism in general is a concept with good potential to be developed in either future expansions or future versions of the game. I would make a tourism an economic component that provides a way for very diplomatic civs to make some money. Of course, I haven't considered a way to balance all of the additional income adding tourism to the game would provide, but I could foresee it working like this:
In the city detail screen, under the trade routes, but above the list of buildings, there should be a screen to indicate income from tourism. There would be no income from domestic tourism, which I know doesn't completely reflect real life, but it allows for the diplomatic aspect of tourism in the game. So, for example, if you build two cities and connect them by a road, you would start to receive a +1 bonus from trade routes, but no change in the tourism income.
To start receiving tourism income, you must enter an open borders agreement with another civilization. Then, you must connect to the other city's infrastructure by road or by water. Once these conditions are met, all cities in your trade network will start receiving a tourism bonus from that civilization. Instead of naming specific cities in the Tourism Income section of the city screen, it would display something like "India +1".
Your tourism income could increase based upon these parameters:
- Additional open borders agreements will increase the number of nations you receive a tourism bonus from.
- City size and economic importance can increase the amount of gold received in a city from a civilization. (Similar to the amount of money in a trade route growing.)
- Having a world wonder in a city could increase all coins generated from tourism by a small percentage. (5% per wonder, for example.)
- New world wonders could be developed and old wonders could be modified to make the tourism aspect more appealling financially.
- New buildings could be developed that could increase tourism income. Buildings that increase accessibility like harbors and airports would increase tourism income by making the city more available to foreign civs. Buildings that increase gold like banks and markets wouldn't need to be modified because they would already be adequate for granting bonuses to tourism income.
- Building railroads or discovering astronomy would drastically increase accessibility of cities and increadse trade route income.
- Natural world wonders could be discovered on the map, settling near these would cause the city to have a built-in 50-100% bonus on tourism. The natural wonder tiles probably shouldn't have any development needed to be done to it, but if a tile is workable it should have a natural economic bonus on the tile. (other than roads, if it isn't a special mountain or special lake or something)
For example, if you find the grand canyon, which would be a tile that I guess looks like a canyon, you probably couldn't work the tile just like if it was a mountain. But settling near the grand canyon would grant that city a tourism bonus that lasts throughout the game.
- Last, but not least, while in WAR, no cities would receive tourism bonuses, making war unprofitable for diplomatic civilizations. If a city has unhappy people in its population, it shouldn't receive any tourism bonus because the city is in unrest. (Of course, just whip those angry citizens and you would get the tourism bonus back.)
- Since tourism would improve diplomacy, game innovations that improve the relationship between diplomacy and tourism should be developed.
And wow, I thought about this way too hard.