This is gonna be a long one
My dream game would be an entirely free form multi-genre game. With a procedurally generated universe, with thousands of professions, lots of races, lifelike AI, good graphics, complete simulation of creature anatomy, a detailed but easy to use editor for creating and customizing everything (similar to spore's but more complex and detailed), a complete simulation of ecosystems and creature life cycles, advanced physics and... Well everything
It would start like an MMO where you create your character. But it would not be just another hero. He/She would be a GOD. You would then learn how to play the game. And when you have learned the game. You would then have your god play a game console. (For joke's sake). And create another character, where you choose race, gender, etc. Then the actual game would start.
You would then choose where in the universe to start (Depending on where your race has colonized). And depending on where you start you would have access to different technology and may or may not be able to travel to space. Then when you start you can work a variety of jobs like: Soldier, Worker and various subtypes. Depending on what job you choose you would play different genres of games. A soldier would play an FPS, a worker would play something else and may end up being the boss, etc, etc, etc.
There are so many more features but i would end up writing a whole book if i wrote 'em all
... Sorry for the long post.