Like Spore, but...
At the start of the game, you would get to pick what kind of planet you want: big one, small, one or a moon, and what chemicals would be in the ground and atmosphere. You start with the simplest form of cell there is, and early choises will affect what course the evolution of your whole planet's life will take. You don't just attach parts to your creature, you choose segments of your body and what you want to make of them (this will be a skeleton, it'll need some way to process food, etc.).
At the beginning you don't control your creatue, it simply acts on it's own according to its instincts. DNA-points are not gained by eating or killing, but by surviving for some time it will gradually build up. Whenever you make changes to your creature (and all changes you make cost DNA-pionts), you get to choose if you want to move the whole species or branch off and create a new one. Branching of into a new species gives you more DNA-points, but creates a smaller group which is more prone to extinction. Not changing for a while can boost up your numbers. Any creatures you leave behind will evolve on their own, although slower than you. You can, however, at any time make changes to any other species than your active one, and so create the exact kind of life you want. If the population is large enough, you can even make changes to the atmosphere (like if you want big insects you get more oxygen in the atmosphere).
There are no clear passages between stages, but eventually you'll want to create brains of some sort in your creatures. After it is complex enough, you'll get more control over your creatures, like their social activities: you can choose if they live in packs, alone or in a big colony. By spending DNA-points in the brain, you'll change how your creatures interacts with the world at large. This will be one of the most complex features in the game, so can get the exact kind of creature you want.
The more intelligent you make your creatures, the more control you have over their actions. Eventually, you might build a society and evolve technology. What kind of society you have will to a large extent depend on how you creatures brain works, but you can also use a sort of evolution here, based on social darwinism. After that it's similar to other simulation games I guess, I haven't thought that far. There would be no space stage, that is not the point of the game.
What would be really awesome with this game are the different kind of biotopes you will be able to create. Several different sentinent creatures livin in peace (or not). Single-cell creatures with a hivemind. A single creature, dominating the whole planet. Sentinent plants. And all the ways different beings would adapt to live together on the planet. You wouldn't be forced to have an intelligent species either, you could just evolve your perfect ecosystem all day.
The basic idea is to be a somewhat realistic life simulator with nothing censored out. You'd even have to design the reproductive organs if you want your creatures to reproduce (so probaly an 18+ rating)
tl;dr : A Spore with an 18+ rating, or at least made for people older than 3.
Oh, and sorry if i missed some spelling errors in there.