My Impractical Dream Game™ would probably be a Halo/Half Life/Modern Warfare crossover fighting invading aliens with the help of space marines that so happened to end up in our time while Soap MacTavish and Captain Price and Gordon Freeman were trying to fight off Russian ultranationalists who turned out to be Combine aliens.
And there would be Hello Kitty in there somewhere.

I think the gist was that the impractical dream games are supposed to be GOOD rather than... well... that

Hello Kitty makes everything better

Some people are just afraid that hello kitty will rape your face with awesomeness

I'd make another Syndicate game, but with some major changes to the gameplay. Yeah, I know, predictable. But anyway, onto the details:
Firstly, you're an actual corporation this time. So you don't have 100% control over any regions at any time and you certainly don't get tax revenue from those regions.
Instead you've got market presence and sales revenue. Market share spreads into neighbouring regions as it increases. You can only undertake missions in areas you have some market share in so you're trying to spread your influence while at the same time maintain and strengthen it in areas you already have some control.
How is this achieved? Through, 'persuasion', assassination, espionage, sabotage of the competition elimination of government opposition to you and so on. Essentially the same kinds of missions in the previous games. Where it differs is in that it's a squad based FPS mode. So you're seeing the cyberpunk world through the eyes of your agents and you can change between them freely.

Hey, I had that star wars game idea years ago! Stop stealing!
I was inspired by the old novalogic games that could link together... it's a shame very few games have followed that. Back when I was thinking of it it was MOO where you could jump into Tie Fighter or Jedi Knight, but of course things have advanced since then.
Acutally, my other always wanted game was also a star wars game. (i used to be really into them, until the sequels).
I always wanted Jedi Knight meets Deus Ex..
A mix of RPG and FPS where you could choose between force skills, tech skills, blaster skills and Saber skills.
It'd be a little more freeform than Deus Ex, in that you'd get your own ship and so could fly between planets at will. (with decent space combat, but also with the interior fully modelled so that you could chill out inside, or that you'd have to run to man a gun turret, or race to try to put out fires or fix hyperdrives. ).
But it'd also have the different paths, varied characters and complex story or deus ex, and things like being able to hack cameras, repair or upgrade weapons, and so on.
Finally, and inspired by Red Steel (or at least what I hoped red steel would be when I first heard of it), it'd have GOOD motion control first person saber combat.
This would't be the flippy kung-fu combat of the prequels, it'd be the proper samurai inspired combat of the originals. It'd have lots of first person animations and interactions (like dark messiah or mirror's edge (but good)). It'd be a case of blocking and circling and waiting for a gap in their defences.
Also, it'd be in the more gritty world of the originals, with scrappy machines and no shiny high-tech stuff anywhere. And it'd have a decent story.

I've always wanted to make a Police Quest meets GTA, but on a better engine, with Sim-like/Ultima touches [everyone has some variables, as age, name, place to live, own stuff].
Stuff would be mostly procedural, with no game the same as the other. The gameplay would run around a 'realtime' cycle, where You'd have to drive to work, get prepared, listen to the briefing, make own notes , and then hit the streets, until Your shift ends, then You could, well, go to a pub and get wasted, if You wanted. The players biggest enemy would actually be the game's possibilities, kinda like prostitutes, looting and gambling in Ultima, where You're supposed to be the good guy, but temptation is on every corner.;)
Apart from that there were a million ideas for a Jagged Alliance like game [because it would be more like JA than XCOM], and some very open and lively RPGs, where You could walk in the night through a forest and witness a murderer stashing a body, not scripted of course.;)
Also, a dream of a Crusader like game, with complex controls and a rotating camera was a dream somewhere there.
Due to the current trend for awesome graphics and scripted content, and since the projects were too big for 1 person to handle, it remains in the dream realm. Oh well, i've survived the early 3D Age Of TPP, i'll survive the Age of FPP, hoping for an Age Of Style-Does-Matter-Gameplay-Does and Action-Is-Not-A-Required-Prefix..

Since this is starting to tip into "Impractical Dream Star Wars Game" territory, I may as well throw my dream Star Wars game out there.
You're in command of the flagship of the Rebel/Imperial/Republic/Separatist (depending on the era) fleet. The entire game plays out from your perspective on the bridge, including briefings and transitions between missions.
It uses impossibly perfect voice recognition software--your primary method of control is by barking out orders (although you can also check various displays and enter orders using your personal command console).
So you might say, for instance, "Comm officer--open up a channel to Rogue Leader," and he'll do just that. Then you give your orders to Rogue Leader, as general ("Engage the enemy") or as specific ("Engage the TIE Bomber squadron attacking the medical frigate--keep shields double front, and don't use your proton torpedoes unless absolutely necessary") as you like. And after you send Rogue Squadron out, you can pull up a live video/audio feed to keep track of their progress and give new orders as needed.
Throw in a ridiculous number of campaigns covering different eras in Star Wars history--ancient Republic, Tales of the Jedi, Clone Wars, the Jedi hunts, Galactic Civil War, the Yuuzhan Vong conflict, whatever's going on in the latest books (I got out of the Expanded Universe years ago, so I have no idea). Every ship ever modeled, drawn, or even vaguely described in some source book is available to you. You develop and assign missions based on the current galactic situation, and the campaign proceeds organically based on your victories and defeats.
And that would be the last Star Wars game I'd ever need. Never mind being a Jedi, Sith, smuggler, bounty hunter, or any of that nonsense--put me in Ackbar's shoes.

This one's not impractical, but I really want a top down shooter with hack n slash action RPG stat building and damage where you run around with WASD and shoot with the mouse. You still have to do all the aiming and running around avoiding things like in alien shooter or those sort of games, but the damage is determined by your item and skill level and all those RPG modifiers. There would also be a skill tree and socketable guns (thousands of different ones for loot).
So I guess it would really be a very traditional diablo clone, but instead of stopping and wacking things you can run around freely spraying bullets everywhere and unleashing all sorts of skills with torchlight-like over-the-topness.
I can't believe it hasn't been done before, it seems like two very easy to combine game concepts that are both incredibly addictive.

Impractical dream game concepts, you say?
I'll start with something that isn't the most impractical by a long shot, though still rather lofty and not commercially feasible. I tend to look at it as an RPG dressed as an FPS, and I mean it in more of a Deus Ex sense than a Borderlands sense.
The backstory is irrelevant and rather cliché at this point. Just know that you end up stranded on an alien planet with two sapient races locked up in some sort of war (not a race war, though; there's many of each representing the various factions you'll encounter), and you have nothing to rely on save some nifty powered armor and your guns.
This isn't your usual scripted game war, though. There's a whole dynamic campaign going on (think Falcon 4.0, then turn it up another notch), with NPC units traveling around, getting into battles, and having to maintain logistics and fend off espionage attempts, among other things, and there will be a noticeable effect if any given faction controls a particular area and its residents. It will be fought whether you actively try to change the tide of battle or not, until somebody manages to win out.
Your powered armor is of utmost importance to surviving here. In addition to keeping bullets from ripping through your meaty body, allowing you to breathe where you normally couldn't (say, underwater or in a poison gas-filled area), and helping you get around easier through augmented mobility, it's also needed to interface with all the local electronics, and-most importantly-translate for you and the locals. No aliens speaking English here! (In fact, they'd speak their own actual fictional language, and the suit just provides subtitles.)
It can also be upgraded in numerous areas to do things such as increase movement speed, enhance raw strength, make less noise during operation, translate better, jam enemy equipment, hack into things, enhance accuracy by reducing arm sway and recoil, and all sorts of other features, both hardware and software.
Be warned, though-your suit CAN be disabled, which pretty much denies you all of those perks (including translation!), though it does help with stealth somewhat. Even if it's not shut off completely, there are several important subsystems that can take damage over time in addition to your meat body having different health counts for your limbs, body, and head Deus Ex-style. (You probably won't take too many flesh wounds and live if they penetrate the armor, and it takes quite a while for the suit to patch you up if it does happen-especially if the life support subsystem has taken damage.)
But most of all, in this war-torn hellhole, you shouldn't expect a mission to go as planned. There will likely be multiple ways to finish the mission, and chances are whoever you're dealing with knows more than they're letting on. Will they really hold true to their word, or are you just another pawn to them? Intrigue and betrayal are aplenty here. (Just wait 'til you meet a certain character and find out why you really ended up here...)
I'm currently questioning whether I should have hunger as a game mechanic, in order to drive the player to make an alliance with one of the factions just to get the necessities of survival, let alone suit upgrades and such. There should be a genuine sense of urgency, but it should NEVER feel scripted or forced on the player.
I'll just leave it at that for now.

Not a game, but a useful (though with current technology, neigh-impossible) mechanic therein.
Instant translation and modification of voice chat to fit with the game's lore/world/setting. I could tell the group's "tank" to concentrate on a specific enemy - regardless of the language, dialect, or even slang I may be using - and he would instead hear the order as my character would have spoken it (probably pretty well-formed language, or slang fitting the time period), in a language of his choice (perhaps I prefer to speak Swedish, but a guildmate is French, and none of us speak very good English) but in a voice of my choice (so I can have the high-pitched voice of the irritating gnome I play, or I may chose to let my friends hear me as I really sound).

Easy....
Shenmue 3 :D

I've got another one...
You start with a game similar to JA2, where you hire a bunch of mercenaries to take over a small country.
However, you're not doing this for the benefit of the people (unless you want to be benevolent), you're doing this for power!
Once you have your first country (perhaps you could pick one from a map?) then it's time to expand, by trade, culture or violence.
Sort of JA2 meets Total War. All the while being careful not to piss off anyone too much bigger than you, cos you don't want to start a World War... unless you can win one.

once the technology is there, and I meet a programmer ready to go to bat with me, it's this, loosely:
A game with no "NPC"'s in the common sense, and death and life are equally important.

That was cruel.
Oh well, at least we'll have Yakuza 3 soon enough (but without hostess clubs--what the fuck, Sega?).