A fucking open world, co-op zombie survival game.
You and 3 other friends. The city starts out normal, nobody is infected yet, and at the start of the game a few people get the disease and it spreads from there. It would basically simulate a zombie apocalypse. It seems that so many zombie games hand you a big gun as soon as it starts and you mow down zombie hoard after hoard, which is fun and all, for a while. I want to see a game where the infection slowly begins to take over, people start to panic, the cops and military come in and try to control the infection but begin to get overwhelmed, cars crashing, people looting houses and stealing, anything to get out and survive, and you are caught in the midst of all of it. You would start off with basically nothing, except whatever would make sense in your situation (such as you could start off in an office, a hospital, ect. and each would influence how you play the game), and would have to gather supplies and weapons before the infection takes over the entire city.
I mean, it doesn't have to be extremely elaborate, with extremely deep AI and intricate physics. At least get the basics down: An open world city, where you can get in cars and drive them, go in a decent number of buildings, have them filled with a variety of weapons and supplies spread around (everything from guns to simple melee weapons), decent AI for the city people (similar to Grand Theft Auto 4, would run away from danger, try to fight it off, call the police or fire department), and a simple "person gets bit, turns into zombie later" system. I was thinking a campaign could be designed around the story, with a mission system similar to Fallout 3 except with the infection spreading and moving around (towns could get infected, fight it off, or be abandoned), and you would do missions, all of which would help fight against the zombie horde in some way.
And then you could have the pure multiplayer part of the game which would be simpler, quicker games. I was thinking of having a gametype where the infection spreads normally in a specific area in the open world map, and you would have X amount of time to get out of the city any way you could before the military nukes the place to make sure the infection doesn't get out.
I could go on with this idea for pages lol. I've put a lot of thought into this idea. But alas, I have no idea how to make games.
And also, a sequel to Shadowrun. One of the funnest multiplayer games I've ever played, despite it's terrible lack of maps and features.