A while back, RPS had a thread where members of the community could describe their vision of an ideal game (http://www.rockpapershotgun.com/2011/04/12/whats-your-dream-game/).  This, interestingly enough, was already on my radar, as I’ve had a few ideas kicking around in a text file on this very subject.  Of course, given the wide variety of game that I enjoy, it’s hard to narrow it down to one “dream” game, so I’ve laid out a few concepts that I’d be very interested to see, as well as the current progress towards them, and which games have come close.  I’m sure that it’s not (yet) a comprehensive list, as both the technology and overall possibilities in gaming are changing rapidly, but I think it’s a good snapshot of what I would look for in an ideal game at this point.

(You’ll note that open-world gameplay will probably factor in quite a lot here.  There is a reason for this: an open world provides for the annoying buzzword “emergent gameplay,” which actually means that having an open world to explore means that you can often find ways to create your own fun – something that also greatly expands replay value.  With a linear game, you play through the levels once, and that’s mainly it – you can revisit them, but you can only do that so many times before you get tired of retreading the same old steps.  Because of this, a game that has open possibilities, whether an open world in an action game or open-ended scenarios in strategy games, can stay enjoyable for a lot longer, as there’s always something new to do.  Beyond that, throw in enough detailed procedural generation that creates interesting worlds and scenarios, and you’ve got a game that you can enjoy in new ways almost indefinitely.)

-A city sim with sandbox mode where you can build your ideal city, and then explore it like you would an open-world game – walking, climbing, driving, flying (helicopter), etc.
-Games which are close: none, really – Sim City 4 had vehicles you could drive, but only from a top-down perspective.  In terms of the open-world aspects, Just Cause 2 is probably the closest in offering the fullest range of exploration.  This idea was first (partially) realized with SimCopter, which would let you fly around cities you had previously created in Sim City 2000.

-A racing game with arcadey, kart-racer-style driving, but instead of being a series of tracks, being set in an open driving environment with many miles of road and interesting places to drive to (either in a realistic setting or a stylized kart-racing setting like sonic & sega all-stars or smk)
-Games which are close: not a whole lot, really.  Recent releases such as Blur and split/second, as well as the mentioned Sonic & Sega racing game, all take place on a preset series of courses.  Probably the game that gets closest to this today, despite its flaws (and somewhat different driving style) would be Burnout Paradise.

-Something along the lines of Saints Row 2, but using complex procedural generation to create an unlimited number of living cities and transport networks between them, complete with buildings, roadways, factions, activities, etc.  Add in a huge amount of customization, a wide variety of vehicles and weapons, and you’d be most of the way there…
-Games which are close: well, Saints Row 2, but with a limited environment.  Fallout: New Vegas, albeit without vehicles, but with the ability to mod/add in tons of new customizations options, weapons, etc.  While a completely different game, Subversion showcases the type of procedural generation technology you would need to generate cities on-the-fly from the ground up.

-A procedurally-generated Myst-like game, less about puzzles and more about exploring beautiful environments.  Think of the characteristic environments of the various Myst games and Uru, with options for near-infinite exploration.
-Games which are close:  Myst games have interesting-looking environments, but limited exploration and environmental scope.  Nothing is close as of yet.  Jonathan Blow is currently working on an interesting game with interesting environments and puzzles that seems a little in this vein, but is still in the very early stages.  Honestly, the closest thing there is today for “explore procedurally generated beautiful worlds” is Minecraft, which produces some impressive vistas despite its blocky graphics.

-Someone, make a sequel to Oni.  Seriously.