My truly ideal game would be a first/third person open world action RPG in a sort of steampunk/fantasy/sci-fi world with all sorts of creatures (humanoid and otherwise)and every sort of weapon imaginable, with several large cities that you could traverse on the ground or rooftop style like Altair, but also open countryside and small towns and such. You would be able to kill anyone (not even that I'd want to, but it's awesome to know that if some NPC frustrates you you'd be able to stab him in the face and then blow up his house). It would feature destructible environments and ragdoll physics of course.
There would be some sort of epic story tying it all together but you could take you sweet time finishing the game. It would be both single-player and multiplayer (Diablo-style) with a world as large, varied and richly detailed as WoW. Side quests would be unique and for the most part require you to actually think and make choices, and not just be kill x monsters and retrieve y items. Enemies would take a long time to reappear, so no waiting for monsters to respawn, which helps with suspension of disbelief. However, in cities and the like, you'd find no shortage of thugs and thieves you could beat up for experience as well as many quests that didn't even involve combat but gave you experience or talent points or some such.
You'd be able to sort of develop your character's own custom class over time, utilizing a variety of different upgradeable talents, and you could choose whether to be a noble hero of the realm or a complete villainous prick. You could save a village from bandits or break into a bank, kill everyone, and take the money for yourself. Then burn the bank down. You'd basically be able to choose your own allegiance this way, but your actions would of course affect your reputations with various kingdoms and factions.
Oh, and your decisions would permanently affect the world around you for the rest of the game. If you blow up a castle, it stays blown up. If a house burns down, it stays burnt to the ground, and perhaps given enough time, a new house would be built in its place. However, this way it would be possible to leave entire cities in ruins for the rest of the story mode JUST BECAUSE YOU CAN.
So basically if you took Oblivion, Fallout 3, Half-Life 2, Assassin's Creed, and the Witcher and crunched em all together to make a big epic mess of a Scifi-fantasy RPG. It would actually be possible with today's technology, it would just require a hell of a lot of work. And the final requirement--IT MUST BE AVAILABLE FOR THE PC.
I'd be swimming in pure happiness for days if this game was ever made.