You know what game I'd like to play?
I like open-world games, from Grand Theft Auto to DayZ (event though those two games are very, very different). I like the randomness that a multiplayer game with dozens of players can produce.
My game would be set in a massive, open world. Parts of it would be carefully crafted so as to be interesting to explore. The maps probably wouldn't be as big as DayZ, and there might be load screens between them, but it'd be less of a burden on the system that way. You could even have the open world around the crafted part be procedurally generated, like Minecraft, but an infinite world in this setting would just make for a lot of wandering around by yourself. Different maps would be released as the game developed, with different settings and environmental hazards. A snowy winter map makes cold one of the hazards; if bandits rob you and take your coat, you may freeze to death. In a desert map, you require more water.
Your task as the player is to survive and thrive, and that's mainly it. The game would take elements from DayZ, MMORPGs, STALKER, and Minecraft.
The graphics probably wouldn't be cutting edge. The models would be cleaner, lower polycount, with painted textures. The idea is to get the highest framerates possible, for the smoothest gameplay, instead of trying to achieve photorealism (most players don't have the horsepower on their machines to play maxed out anyway).
For immersion sake, the game would play entirely from the first person. You'd walk, shoot, drive, and even fly aircraft from the first person. As many movements and motions as possible would be animated, like in Dead Island and similar games. You wouldn't press USE and magically appear in a car. Your character would actually open the door and you'd climb in. You'd actually grab onto and climb ladders, push doors open, things like that. When you look down, you see your feet.
The environment would not be static. Ideally, it'd be destructible, like in Battlefield 3 and 4. Buildings and objects could be moved, placed, or destroyed, all modeled with a believable physics engine. If you ran through a room full of bottles, cans, and trash, these items would get kicked around and moved as you bumped into them. You'd be part of the environment instead of a pair of floating hands on a completely static backdrop. Can't get into a building? Blow a hole in the wall.
Your character would be extremely customizable. Pick your sex, pick your skin color, build, height, weight, eye color, hairstyle, all of it. Pick out your clothes. You could even be LEFT HANDED. Your character would be YOU, if you wanted. The body type and size would, to some extent, affect your character's ability. If you made a big fat guy, you wouldn't be able to run as long. If you made a waifish, thin woman, you would be more encumbered when carrying a given load. You could be a short guy to be a smaller target, or a big guy to be a little stronger.
When you start the game, you first create your character. You start off with a certain amount of in-game money, which you can use to equip yourself from the available starter equipment. You may forego the binoculars to get more chemlights or a better flashlight, for example. You may spend some of the money on a starter pack or utility vest so you can carry more equipment.
Items in your pack are not readily accessible. When you hit the inventory button, your character removes the pack, takes a knee, and opens it. Don't put stuff you need (like your spare ammo) in the pack.
After you pick your starter inventory (which does not include a firearm, but does include knives, hatchets, and other tools), you're inserted into the game world through one of multiple entry points. The setting is a huge quarantined area. Most of it is open country, but there are villages, towns, even cities. The quarantined areas have been attacked and claimed by various large (100+ feet tall!) kaiju-type monsters. These creatures come from the sea, and would occasionally make an appearance in the game. They're not the main threat, though. The main threat are the smaller creatures, the mutants, the parasites, and other people.
Monster parasites would bond with animals like people, bears, wolves, etc., to make hostile and dangerous mutant enemies. So why go into the quarantine zone? Because Kaiju and mutant tissues are valuable. Other artifacts and items that appear in the game world are very valuable. If you find one, and make it back to the entry point, you're able to sell whatever you collect for cash. You can then upgrade your character, buy better gear, buy weapons, and go back in better armed.
If you die, you lose everything you're carrying. The stuff you have saved in your character vault is preserved, though.
There are NPCs in the game as well. Random people, scavengers, bandits. Some ignore you, others are hostile. The quarantine zone is also patrolled by blue-helmeted UN military forces. Depending on the map or server settings they may shoot on sight, and may even send aircraft over that attack all players. You may be able to surrender to them, in which case you'll lose all of your gear and will be escorted off the map. If you ambush and take out a patrol, you'll be able to score heavy weapons, night vision, stuff like that. But it's very risky to try.
You can also spend your money on abilities for your character. Buy the equestrian perk and you can ride horses, if you find one. Other perks would allow you to reload faster, use certain weapons better, run faster, carry more of a load, fly aircraft, or be a bit more durable.
The downside? If you die, your character loses all of your accumulated perks. (If you surrender and are escorted off the map, you lose your gear but not your skills.) None of the perks will give you a massive advantage over new players, though. It's not like an RPG. There are no levels. The new guy with a crappy pistol can still kill you by shooting you in the back of the head.
There are tons of weapons. Melee weapons like knives, tomahawks, and swords, for example. There are also improvised weapons that you can find: crowbars, shovels, 2x4s, etc. Many of these big items can't be stored in your inventory or magically stuck to your back. If you deselect your 2x4 you just drop it. You can also throw objects to cause some damage. You may be able to kill an enemy player by dropping something heavy on them, for example.
There are also lots of guns. The ballistics aren't tied to the gun so much as they are the ammo they use. Rifle rounds will fly far and penetrate through a lot of stuff. Pistol rounds move a lot slower. The game features basic ballistics, so that bullets drop along a trajectory and actually have flight time. The guns aren't real-world examples, just fictitious stand-ins to give the player a wide variety: hunting rifles, pistols, machine guns, assault rifles, battle rifles, lever rifles, pump and auto shotguns, magnum revolvers, etc. Many of the weapons can be upgraded if you make it out of the quarantine zone and back to your gear vault. You may also be able to find an arms dealer/gunsmith NPC in the game that will do this for you. They can add scopes, lights, sell you bigger magazines, increase rate of fire, etc.
The game would be the most fun you could have sitting in front of your computer.