I'd like to revive the old idea of
Natural Wonders.
These would be like unique super-resource tiles with individual graphics, scattered fairly randomly across the world (but each one located in a specific terrain type). Some would occupy 2 tiles, or even 3 on a large/huge map. Their names would flash up on mouse-over
When located in a city's workable area, a Natural Wonder would have the basic properties of its underlying terrain plus 1

, and immediately give +3

for that city; those involving animals or fish would also have a

bonus, like regular food resource tiles. From the start of the industrial era it would generate a further +3

in tourism, increasing to +5

under Environmentalism. If located within a civ's cultural borders but
not in working range of a specific city, it would provide a general +1

& +1

in
every city. A player would therefore have to decide what is more beneficial - big gains for 1 city, or small gains for all cities.
Roads could connect up a Natural Wonder to enable the tourism revenue, but destroying it with terrain improvements such as mines, farms or towns - or by actually building a city on top of it - would not only anger your own population, but incur diplomatic penalties with other civs.

Having a natural wonder near a city would make it a more tempting target for invaders; fighting battles there would, conversely, increase your enemy's war-weariness.
Suggestions for Natural Wonders:
Serengeti - plains, 2 tiles, multiple animals graphic;
Ngorongoro Crater - hills, 1 tile, multiple animals graphic;
Kilimanjaro - plains, 1 tile, green shield-mountain with white peak graphic;
Okavango Delta - desert/river, 2 tiles, reeds & animals graphic;
Great Barrier Reef - coastal, 2 tiles, multiple colored fish graphic;
Ayres Rock/Uluru - desert, 1 tile, eponymous graphic;
Mount Everest - mountains, 1 tile, extra-high peak graphic;
Lake Baikal - hills, 2 tiles, deep/dark water graphic;
Grand Canyon - desert or hills, 2 tiles, eponymous graphic;
Yosemite Valley - hills, 1 tile, sheer rockfaces graphic;
Yellowstone - mountains, 2 tiles, plateau & geysers graphic;
Everglades - coastal/grassland or forest, 1 tile, sawgrass and alligator graphic;
Niagara Falls - hills/river, 1 tile, cascade graphic;
Angel Falls - mountains/river, 1 tile, cascade graphic;
Pantanal - jungle/grassland, 2 tiles, pools and colored birds graphic;
Galapagos - group of 3 or 4 1-tile islands, cactus-tree, tortoise & bird graphic
Grand Banks - coastal, 1 tile, multiple fish & whale graphic;
Bay of Whales - coastal/tundra or ice, 1 tile, multiple whales graphic.
Here's the
original discussion on this subject.
Anyone have more suggestions? (If somebody fancies creating a mod with Natural Wonders, I would be happy to design the graphics.

)