Okay, so here is what I think about Natural Wonders right now:
- Natural Wonders are always placed on geographically accurate locations on the map. Not all available Natural Wonders will be placed on the map in every game, however. The placement logic will make sure to spread Natural Wonders roughly equally between continents. Some Natural Wonders of the same type (e.g. mountains, volcanoes, waterfalls) can be mutually exclusive, so that never all of them appear in the same game.
- Natural Wonders do not inherently start out revealed. Some Natural Wonders require a technology to be revealed. If you have the reveal tech, or if the wonder does not require a tech to be revealed, it will become visible on the map when a Scout or Explorer moves unto the tile or it is covered by your culture. If you have the reveal tech, Natural Wonder visibility is traded along with map trades.
- Natural Wonders can be explored. All Natural Wonders require a technology to be explored, although sometimes the explore tech can be identical to the reveal tech. If you have the explore tech, you can explore a natural wonder by moving an Explorer into the the tile. The first player to explore a Natural Wonder will receive a bonus. You do not know if a Natural Wonder has been explored yet or not, but you will receive a notification when someone you have contact with explores a Natural Wonder. Natural Wonders that are connected cannot be explored anymore.
- Natural Wonders can be connected. Natural Wonders are able to be connected if they are on a workable tile, i.e. they need to be in your city radius and covered by culture. If that is the case, the working city needs to fulfill a requirement that is specific to the Natural Wonder, for example build a specific building. Once the Natural Wonder is connected, the city receives a bonus. Connecting does not require a specific tech, but sometimes it is indirectly limited by techs that unlock the required building etc. Natural Wonders that have been explored can still be connected, however Natural Wonders that are already connected cannot be explored anymore.
- Natural Wonders can be improved. This is essentially unrelated to Natural Wonder specific mechanics, but certain Natural Wonders can receive improvements. Common improvements for Natural Wonders are Nature and Marine preserves, but it is also possible to add Pilgrimage Site or Research Station etc. as Natural Wonder specific improvements.
- Natural Wonders can become obsolete. If a Natural Wonder becomes obsolete, the effect from connecting the Natural Wonder expires. However, the Natural Wonder is still considered connected. Obsolete Natural Wonders can still be explored and improved.
- Natural Wonders may influence terrain yields independently of all the above mechanics, especially in combination with some improvements. Techs and buildings (especially Tourism and Hotels) can increase the yields of some Natural Wonders further.
- How can maritime Natural Wonders be revealed/explored? Currently leaning toward: Caravels or any ship carrying an Explorer.
- How can impassable Natural Wonders be revealed/explored? Some Natural Wonders are peaks and therefore impassable. I don't think it's satisfying to make those Natural Wonders passable to units that can reveal them, because before they are revealed they look just like (impassable) peaks, so it would result in tedious clicking on various peaks to see if they are secretly a Natural Wonder. Current idea: impassable Natural Wonders are revealed as soon as they are in line of sight of a Scout or Explorer. Since those Natural Wonders are usually mountains, that is plausible. Revealed Natural Wonders become passable to Explorers so they can explore them.
- Should there be a late game Explorer unit? What would it be?
- Can similar mechanics be applied to cultural historical research and archeology? We talked about prehistorical sites and city ruins before in this context.
- Almost all of the above is very close to resources, so that will be the way to implement Natural Wonders. This also means that Natural Wonders cannot share their tile with a "real" resource, though.
- The only downside I can see here is for interconnected Natural Wonders that span multiple tiles, like Pantanal and the Great Barrier Reef. In that case we could still place special features with unique art on the tile, along with a (maybe invisible) resource. In any case all Natural Wonder mechanics should be implemented with resources.
- Connecting a resource to a city is easiest implemented as a "quest" through the event system. Natural Wonders could tie into the quest system in other ways too, and the "holy mountains" quest should be removed.