I know that it would be impossible to make now. But I also liked the idea of an open world game with the entire earth (and maybe the moon) as the open world. I'm not even talking about a rough scetch of earth with a couple of noticable cities, I mean detail every house, building, farm, and skyscraper on the planet, maybe using google earth or something to detect any changes to the world (the construction or destruction of buildings)to update it on a regular basis.
Then add some ridiculous toys. Flying cars, jet packs, teleporters, etc. And then put in some secrets for the player to find (Go underwater in a sub and find the city of Atlantis, or blast off to the moon and build a colony). Add some RPG features like level ups for combat, engineering, physical strength, mental strength, influence on people, etc, etc.
Finally, create some quests. Building a rocket for example requires you to have a high engineering level, as well as a strong influence on people (convinsing NASA to help you build this rocket.) and of course a hell of a lot of money (mostly earned from other quests). And your reward for completing this task is the ability to travel to the moon.
yeah, such a game is probably never going to happen. but you can dream right?