Hmm.. most folks on this thread want bigger sandboxes with more stuff and freedom, or a direct sequel/update to something existing, in both cases with huge replayability. I guess that's to be expected when someone asks for a "dream game" (singular) in PC Gaming. I think better sandboxes are good, but those will get made in any case, it's just a matter of time.
Some of the stuff I want to see:
Some emotional weight in an otherwise mass-market, high production value game. It's impossible to be brave in a game if the game doesn't offer the choice of being cowardly. It's impossible to be a good guy if you can't be a bastard. By taking away all choice, COD, MoH and so on reduce duty and honor to a *backdrop* instead of enabling the player to *act* those things. Even something as simple as civilians sprinkled all over in select places in COD (taking realistic damage) would go a long way. There doesn't even need to be an in-game reward or punishment for it. Some people, knowing there are no consequences, might choose to roleplay a war criminal, not identifying their targets, or even shooting civilians on purpose. Some others will roleplay a respectable soldier and check their targets. Yet another group of people will simply ignore the civvies since they do not have a bearing on advancement in the game. This is just one example, of course. The real point is that even a touch of adult themes in mass-market games would, in addition to making that game more memorable and meaningful, pave way for games that hinge on adult themes - something we now sorely lack.
Tactics/action game with a strong realtime management component layered on top. Some kind of system to balance or adjust to support what you want to do. X-Wing was like this, but I'd want the management component to be more demanding, offer more choice and reward more for thinking forward. Special abilities with load times and cooldowns. I've never played mech games, but I think a fast-paced, small urban mech game where you'd use buildings as cover and climb them, bust through walls and so on would be a great shell for this kind of mechanic.
A parkour game. I just noticed something like that, called Mirror's Edge, is already on the way.
An arcade shooter, a little like The Club, but much more over the top and fantasy-like. You'd build momentum/power with comboing, and fast time would be the primary goal. The game would morph as you get better at it: first it'd be about survival like normal FPS, then about clearing obstacles efficiently, and finally you'd sort of race through the maps. I'm talking about the kind of power difference where after spawning, you'd need to headshot the weakest enemy to bring them down in one hit, and at top power you could just run through them and they'd explode. The best tactic at every individual obstacle would depend greatly on what you have managed to acquire in the same level before that obstacle, keeping the gameplay fresh. A good example of that sort of system is Tumiki Fighters.