One of my earliest ideas was similar. It was inspired by Streets of LA and GTA. A sandlot game with far too many mini-games that let you be either a criminal, a regular joe, or a cop. I called it American Currency and the experience included necessities as detailed and integrated as dressing properly, preparing and eating food (unless you go to fastfood/restaurants/buffets), exercising (all aspects inspired by San Andreas) and obtaining work, shelter, and transportation. There would be opportunities for employment everywhere: honest work, seedy work, and general favors. There would even be a prison life wherein the prison would behave much like its own city which you are unable to leave without being labeled a Fugitive (though with the right connections or trades, you could change your identity).
As I imagined this as a pure sandlot game, it never occurred to me that it might want a story. I figure your character's lifespan is the story, you keep playing until your PC dies, either by design, accident, or old age. Unlike GTA wherein you can't die (so part of the fun is to see how many different ways you can kill yourself like in Groundhog's Day), AC only allows your character to live through physical traumas on a slim luck factor, requiring you to play carefully or skillfully. Kamikazeing in an airplane would have no survival rate... so don't do that.
I've also already mentioned in these forums about four hundred times my idea for a Battlefront version of Final Fantasy. We were never given the full experience of full-scale war although we've glimpsed it plenty enough (foremost in my mind, Balamb versus Galbadia). Herein you would be allowed to play not only as any character from the series, but any grunt or monsters of a given army. Depending on the era of the campaign, you could play as different versions of characters from their past or future (as available in canon). My first example being VII naturally, you could play as MP Cloud, Zack, Cissnei, Sephiroth, Genesis, Angeal or any of the available ShinRa classes (various MP classes + SOLDIER 3rd-1st) versus Godo Kisaragi, Shake, Staniv, Gorki, Chekhov or any Wutai classes during the Wutai campaign. You could play as "Ex-SOLDIER" Cloud and every member of AVALANCHE including Biggs, Wedge, and Jessie (and perhaps other "friends", i.e. Johnny, Dio, Big Bro) versus ShinRa, the Turks, and Sephiroth (and his clones) during the Promised Land campaign. Then later could play as the series mainstays and the WRO versus Kadaj, Yazoo, Loz and the DGS during the Rebuild campaign. Of course, the Turks versus AVALANCHE and the Ravens during the Before Crisis campaign. and of course, that's all just for one part of the game. In the Final Fantasy II battles, you could play as Leon during one era and the Dark Knight during a later era. Perhaps the most exciting part about being able to play army to army is that any vehicles used in the games would be at your fingertips. Magitek armor, Chocobos, Buggies, Cloud's Fenrir and Daytona, airplanes, airships, boats, submarines, sleds, etc.
A game that I'd really like to make though is simply one that promotes this story I'm writing. I came up with a format for the game a long time ago. The gameplay shifts styles multiple times, from chases, to stealth, to one-on-one combat. There's a part early on in which you must sneak through a fortress before being forced to battle in the dark against an opponent with night-vision. Later, you steal a car and chase down your next opponent who then traps you in a virtual world that plays like an RPG. Then there's the battle in the sky on a series of floating rocks you must be careful not to fall off. There's also a character who uses the flora of an entire forest as a weapon.