Oooooooh man, get ready for a novel answer.
Literally:
I'd want a sandbox game, cityscape, changing environment. We're talking weather, main characters, new buildings being made, very realistic.
You can drop into existence anything you wish. You want a car, but you're on a high rooftop? Not a problem, bam, instant Camaro.
And tweaks, as for the laws of physics. Zero gravity, for example.
You can simply do anything, with a reacting environment.
Also, you can walk through EVERY DOOR. Ugh, pet peve, seeing a door you can't go through in a video game.
I know I know, there are games LIKE this, but I want one even better than THAT ^^^^^^^^^^. I want something truly unlimited in potential. Life without boundaries, captured in the essence of a video game.
Again, I know I know, all in good time. Technology is advancing every day, it's only a matter of time until a game like this one can be created. But the question was asked, so I gave an honest answer!