I'll start with an idea I've had for a while now, utilizing the CryEngine SDK, I would create a world based off Wellington City, using google maps I could copy the street layout and if a building is an apartment, cafe, mall etc that gets put there in the game world with an altered name and logo(avoid legal stuff).
There is no real objective beyond living day by day as in you need to "obtain" money which will be deducted automatically every week since you need to pay (the constant costs, relatively unchanged by player)rent/morgage/ power/gas/water/internet (variables, so the more you use them the more they cost but they still have a base cost even if you don't use them). Money is also used for buying food, materials(wood, metals), various furnishings(from beds to computers) and equipment/weapons from axes and knives to air rifles, or if you can acquire a liscence shotguns and MSSAs(assault rifles all limited to 7 round mags, unless you make custom mags, which you do not want the cops to discover).
Money can be "obtained" by selling various items to pawnbrokers, car dealers and recyclers(raw materials can be reduced from electronics, and being almost no risk for the selling goods of disputable ownership). Money can be earned through having a job which will consist of a steady pay but market conditions can change every day,(also it's not a good idea to steal from the business, whether through fraud or actually breaking in).
Combat (3rd/1st person)
Combat can and very likely happen(although chances are higher in alleys and empty streets at night. Essentially everything is a weapon from the sword to the mighty pen(guns are very difficult to obtain and is not advised to walk around during the day with a rifle).
Carrying a laptop home at night and some guy tries to mug you? Bash him over the head a couple of times and he may just run off. Or he may redouble his efforts and kill you.
The health system looks similar to Op Flashpoint RR so you can see your current limb damage, but modified so you also see other factors- stamina, blood and pain tolerance. Stamina will remain at 100% for the first minute(adrenalin) of combat but will slowly decrease, dimishing the amount of damage you deal through melee combat and your sprint distance.
Pain and blood(to a lesser extent) also do not come into effect until the first minute is up but effects combat much more greatly, say you get cut adrenalin negates that for the first minute but if you get cut again not only will you be bleeding potentially twice as fast(cut depth a a graze has no bleed effect but a 3cm deep gash in the leg is a different story) it is possible to bleed to death if you do not seek medical attention(self healing is limited), but also you can enter a "rage" mode where adrenalin comes back into effect instantly restoring stamina to 100 but at the cost of limited vision and no finesse skills(more fisticuffs than karate).
Pain can cause fainting/blacking out and is more about received damage. Say you break an arm, you can no longer use said arm in melee combat, no more lifting large objects, no dual wielding and 2 handed weapons are restricted by size a rifle with a strap around you will have severely reduced accuracy, a two-handed sword will deal less damage and take longer to swing.
Eating and drinking is essential to life, so you'll have to buy food every now and then but unless it's canned, it will rot. Also sleeping regularly helps maintain stamina, but if you don't your skills will increase at a slower rate and you may lose a few points.
Crime and Punishment
Theft is a crime and being caught will put you away for a few years and incur loss of property, being caught you would lose your awesome apartment, everything inside and money down to $5000, a crappy low paying job, harder to get new jobs, increased expenses and worst of all a decrease of skills. Theft, Assault and Murder, are the main crimes, having escalating skill penalties. Theft can be found out if you sell large quantities of (item/s) to the people and the may get suspicious.
Certain actions may require a minimum skill level such as lockpicking you mad need atleast a skill of 20 to attempt to pick a lock but if you have 40 the success rate is increased by 50% and at 60 your speed is doubled with 99 being the maximum and cracking it almost instantly. Performing a skill action will increase your skill level. Ever wanted to max out your stats? Well then sign up to a gym, there you can increase your strength/speed/agility, but you can buy certain pieces of equipment to increase them but never expect instant results. Skills have to be maintained if your skill is higher though it will drop at a slower rate, so if you don't pick any locks for a while expect to start forgetting. Internet, yes there is a reason for it beyond entertainment(of varying forms) you can increase your skills but will be over the process of a week(capped to level 50).
A job can be where your character spends most of his/her day at, working hard to get money, but it can get rather boring at work...
Not all jobs are exactly legal, these range from smuggling, item/vehicle retrieval to planting (mostly false)evidence, "teaching someone a lesson" (vandalism/assault) and assassinations, the last two can only be given by gang/mob/mafia members.
That's right make your own character, but also their past effects their skills so if they were a sporty kid then they get a boost to speed etc.
That's right want to rob a bank, but don't have the skill/resources or need someone to watch your back? Take it online or to a LAN.
How would I achieve all this, well it being my dream game it would do a lot myself and work on it in my free time and I could recruit some modders to help work on textures and programming. How would I sell it? I wouldn't I would start a kickstarter for donations and give it away free to the world(with those kickstarter guys getting their hands on it first and to ensure future support). Am I going to far, hell I am but this is a hypothetical game.