Well there's two that I would really want to make.
The first would be a massive MMOFPS with very heavy rpg elements thrown in. Only with player made cities, battles for resources, technology development, huge freakin' vehicles, and AI controlled NPC's to round out the numbers for a truely war like feel. You choose from one of three nations in the beginning, one that uses biotechnology (implants are actually mutations, vehicles have an organic feel to them) one that uses cybernetics (just like it says, deck yourself out in metal and blast away) or a magitech one (weird magic as science stuff). If you're a good enough player then you can move up to commanding ranks, which, once there, you get to actually make up the missions and eventually the battleplans that other people take on as 'quests' of a sort. For instance, let's say that you're at the rank of a General, you can make the order to capture an enemy city, and then with that someone down the line, let's say a Colonal, he makes the decision to order some of the men under his command to capture a power reactor to facilitate the taking of the city. This order is related to the Sergeant in your squad, he makes the quest handed to you to capture the guardposts around the powerplant. You see, each higher up can make up quests that can be recieved by people lower down the food chain, all based on their own quests, and everybody gets rewarded when it works. You get rewarded for neutralizing the guards, your Sergeant gets rewarded because he helped to capture the power plant, his Colonal gets rewarded for capturing the city, and the General gets prestige for his plan working. Now along the way they'll be NPC's and PC' both working to accomplish these goals.
You also have to pick a class. Something loose, little inspired by the current choices offered by games like Battlefield. You know, sniper, engineer, that sort of thing. That's not to say that you can't snipe as an engineer, you just won't get the cool sniper abilities that the sniper does, but he also won't get the awesome abilities that you do. I'd also go with a skill tree for classes, just something to have people kind of try to specialize in soemthing so that everybody can be a little different. Like engineers being able to focus on vehicle construction, building construction, or weapon construction.
Yeah. Anyway, you can also opt to either join with the military of your chosen country, or join one of the companies that does the research and development for the country. This part becomes a little Civ IV-ish, wherein the more advanced your technologies, the better your troops will be by comparison. You can also eventually go freelance and either become a mercenary working alone, or form your own mercenary troop, and then pick and choose your battles based on bounties offered by officers. Mercenaries are responsible for the upkeep of their own gear, but are able to customize it in ways that military and company soldiers cannot. You also have the option of staging a coup or creating your own breakaway republic for others to join.
As for the nations, well, both the starting ones and the ones made during the game will have PC's running them eventually, which, for these players, makes the game a lot more like an RTS or Civilization building game, where they have to choose sites for cities, building defences, tax rates, that sort of thing. They also have to option of choosing their form of government.
Then there's the vehicles, which range from battle wagons and your standard fare of planes and helicopters, to stuff like the TITAN's from Battlefield 2142. All fully pilotable, with NPC's to fill out crew rosters if need be.
Huge huge undertaking, with hundreds of players running assaults and sieges, with thousands of NPC's being controlled to keep everything running in the background. Alliances can be made and broken, nations can be formed and then either conquered or destroyed. Hell, there's a chance that one nation could actually manage to win the war and control everything. Either way I just want to see it happen.
And while we're at it, throw in a mech or two.
Speaking of which.
I'd like to make a kind of sequel to R.A.D. for the PS2. Only, let's make it as anime inspired as possible. The game would play out a bit like City of Heroes/Villians where you would create your character and choose their powersource from either a powersuit, psychic abilities, magic, genetic modifications, cybernetics, or chi abilities. Then you choose which foundation you're the head of, each one has a focus on a specific kind of techology, which would make later upgrades cheaper or more expensive. You then choose your vehicle, from say, for instance, a kind of personal mini-jet, to a special motorcycle, from a magical horse like being, to a small robotic dragon. Finally your choose your method of giant monster fighting, which boils down to whether you want your own giant monster, a giant robot, or a giant magical golem. Each choice there has a subchoice which boils down to do you want it to fly, be good on land, or more water based, and then that breaks down to a choice of the Fast, Powerful, or Average emphasis.
The game itself has you running around a huge map with several large cities and plenty of varied landscape between, all trying to stop everything fron alien invaders to ancient demons. Some battles will have to be fought by you personally, but sometimes the enemy will bring forth some type of giant monster or machine, forcing you to call out your own. In these giant battles you can switch from taking direct control of your own giant fighter, to running around as yourself trying to solve everything from hostage situations ro trying to find a way to give your giant and upperhand, all while your giant and the enemy creature duke it out, potentially laying waste to the fully destructible cities and environments.
There's also some heavy RPG elements here, where you choose to improve yourself, your giant fighter, your vehicle, your companions, and your company. There's also some party management, where you can meet other people, befriend them, build a relationship, and eventually reveal your secret identity to them (you are basically a superhero here afterall) and potentially get them to join you. These 'party members' have their own predispositions towards what they can do, meaning that Jim is a better magic guy who's really good at controlling a golem type fighter, but that doesn't mean you can't just make them whatever you want, i.e. screw you Jim, you're a psychic who flies a giant flying robot. This comes into some heavy play when you start having to face more than one enemy giant at the same time. Of course you can also switch to controlling this other person and their giant, or you can have them combine their fighter with yours to form more powerful beings. You can eventually build your own team of heroes, though you can only choose two to take out with you at any given time.
So yeah, basically you take every sentai type of team ever made, every godzilla flick, some DBZ for kicks, GTA, Armored Core, and a dash of Monopoly, throw them in a blender, and you get it.