I'm currently at the Game Developers' Conference. Sitting in a conference room, a lecture about to start.
The previous talk I was at was given by Will Wright, creator of Sim City and The Sims. He's been working on another simulation game, but this one goes from the level of bacteria (with a Pac-man style of gameplay), evolving up to creatures that fight eachother (a combat/levelling up gameplay style that feels like Diablo) to a tribal management game (think Populous) to a city building sim (SimCity) to a nationbuilding game (Risk, Civilization) to the solar system, the stellar neighborhood, and the galaxy (think of Master of Orion, X-Com, and every Sci Fi movie you've seen from "The Day The Earth Stood Still" to "Close Encounters" to "X Files").
Any one of these games seems engaging, but the scope is amazing. Add to this the flexibility given to the player to author and own their own creatures, buildings, and vehicles, with an automatic asynchornous method for sharing content between players - watching this demo (with very little exaggeration) HURT, it was so amazing.
I understand he'll demo it at E3, which signals to me that it's actually going to turn into a game, and not just a tech demo. Look for it on shelves someday - it's going to be amazing.