A dream-game for me would be an immersion-focused proceedurally generated medieval fantasy survivalcraft action RPG sandbox, with simulated kingdoms that, on at least some level, provides simulation for every individual in the world (or at least the ones that become distinct by the player interacting with them). The player would create a character as a child, play through that character's life, and then if your activity included having a child of your own, you would play as the original character's child when the original character dies, and so-on. It would include a broad field of subjects for your character to study. It would include robust physical simulation of a variety of substances, both real and fantastic, as well as have design features for the player to use the given substances to construct anything they can image. It would include far too many places to go and things to do than any player could ever actually achieve, just like the real world. The point of it would be to scratch the exploration itch that humanity suffers but our generation can't relieve. Immersion is an element of design wholly lost to the videogame industry these days. AAA companies focus their design effort on brainwashing and addiction strategies to keep gamers A.) playing and B.) buying more to play. This would be a game that discards all established conventions of 'successfull design standards' and effectively invents its own virtual universe.
When I was young, when I dreampt this up, computer technology was inadequate to make it possible. These days, however, I don't know. We might be there, or at least getting close.