Play a roguelike game in multiple eras of a planet's growth.
No illustrations this time guys, sorry.
Okay, this is a pretty simple concept based around procedurally generated content, meaning that while the concept is simple, the implementation is not. Basically, you begin with a world in an infant state - a Pangea, if you will. From there, the world advances through epochs, with world changing over time, civilizations being born, going to war, earthquakes along the tectonic plates, volcanoes erupting, polar ice caps melting and raising the sea level, so on.
That in itself would likely make an interesting screen saver. However, there are two twists to this idea. The first is that it is a roguelike. The second is that it is a collectable card game.
Each epoch, you are allowed to create a character inside the world and go adventuring - kind of like Dwarf Fortress' adventure mode. The first epoch, you might play a caveman, while the second a barbarian, the third a feudal lord, the forth a soldier in a modern military, a spaceman, whatever. You know. Your typical Civ2 epochs, but playable as a roguelike.
As you are interacting with each era of the game, you may cause changes to the world around you. For instance, you may end up causing a war between two nations. Or you might hunt all the buffalo in an area. Or you might unite all of the world under one ruler. A city today may be ruins tomorrow, so what you do in that city could ultimate affect what kind of ruins it becomes.
Between each playable section, the world advances from one epoch to the next. During this time, you play a card game that will affect the world. For instance, you might have a card that causes an earthquake somewhere (you'd have to select a specific location along a faultline) or a card which introduces a plague to a civilization. Of course, it might not be all bad. You might have cards which can affect a society's growth, like giving them philosophy or a mechanical visionary, giving them access to farming machines ahead of other societies.
These cards come in collectable sets and are built into decks for each epoch. As you play the roguelike aspect of the game, your rewards for exploration or whatever will be cards for that particular epoch (that can be put in decks for the next play through). It would be grand if it was in some way related to your activities. So doing quests for townsfolk will yield cards that improve society and exploring caves will give you geological information, tools, and events.
These cards control not just pressures on the world, but also what is in it. In the early epoch, one essentially designs the pangea through this card system. One might decide to start a tribe of goblins (who, as a race, have stunted growth in culture and society) or one might decide only to include cards that populate the planet with humans. One type of human tribe may require two rivers and fertile farmland while another is nomadic in nature and can only be placed in the plains where a specific type of beast is omnipresent.
This idea really comes from the idea of micro and macro pressures to a procedural content system. Left alone, a pangea world grows the same way every time. But introducing a single pressure could affect that growth considerably. These pressures can be great (earthquakes leveling a society's capital) or small (raiding a dungeon and killing off a group of goblins), but either way, the world will grow according to how you nudge it. So you've got both micro and macro goals for how to do this. On one hand you want to affect each epoch (maybe you want a post apocalyptic future or an Asimovian one of science or... Morlocks - you know who you are), and on the other, you might just be interested in creating a cool playground for your knight or spaceman to explore and interact with.
This is something of a BIG idea, as far as scope is concerned, but Dwarf Fortress has made some strides in the concept of a world with a history, and things like Civilization and Age of Empires have taken the tech trees and epoch advancement stuff and made them reasonable. I don't think this idea is impossible, but it would require epic procedural content... but then, isn't that the promise of procedural content anyway?