A huge sandbox game set in the ruins of British society. Features Havok, Euphoria and Digital Molecular Matter similar to Force Unleashed, in addition it features Volition's own Geo-Mod 2 engine.
You start the game in a bin piled under a bunch of charred corpses and debris (a major fire has burned around here and the debris, while cooked, shielded you enough) by the husk of a grand theatre. The game prompts you to hit a button and you burst out from under them, which shows off the physics and stuff.
The 'city' is large (for games), a little bigger than Liberty City in GTA IV if the city only had one river and all the islands were otherwise closer together and more or less fully connected by land. Outside the city is the countryside, which extends a few more kilometers out from the city and in places is crisscrossed with deserted motorways, in others there are towns and villages, farms, fields, and some forested/moor/valley 'remote' land - all becoming a bit wild and overgrown.
The Infected are all over the place, you can hear their howls from across the city, hear the occasional ratatata of gunfire from afar. Obviously there are survivors, some lone rangers, some family groups, some roaming gangs and some organised military/police groups. Some are good, some are bad, but everyone wants to survive and get out - and that's what the story's all about.
You'll get involved in all sorts of actions - basic survival, food hunts, fights with groups, fleeing hordes of infected, fighting them if you manage to find good enough equipment/people, finding places to call home and then gathering up the materials to fortify them. Maybe you could even band with other people - the more you have, the more you can do, the bolder you can be, but the chances for betrayal if someone gets bitten also rise.
The weapons you find are the standard fare - gangland sawn-off shotguns and uzis, handguns, police issue batons and tazers, black market weapons from around the globe, and, yes, military issue gooseberry fool on the order of several grades of rockets, grenades, flamethrowers better guns and so on.
Other things in the game would include would be a customisable armour system (choose what sort of armour to have and where, maybe combine them at the expense of speed or maneuverability, or remove for the sake of speed), persistent world damage, a raised chance in Infected-heavy areas of finding one of them in the back seat of a vehicle you decide to hijack.
A 24 hour day clock thingy would also be included, and there'd be four weather pattern sets - one for each of the four seasons (a different set of weather states are active depending on the time of year the game detects your 360/PC is set to, or maybe the game could ask you to set the current day of the month and it'd do calculations based off that).
Edit: On the subject of the player being bitten there could be, on the easier difficulties (or after you've found certain locations in the region), a timer lasting between a minute and five minutes, demanding that unless you get to a medical research centreor hospital and inject yourself with a random unmarked needle full of serum the boffins forgot to label, and get miraculously cured, your health meter will dwindle as time passes until the timer expires and you, for all intents and purposes, die.
Edit: There could be multiplayer modes including one where a random player is Infected and has to spread the RAGE about (TimeSplitters-style), a Horde-style mode where you have to work together, find a defensible position and await the masses (the cries and whoops of the infected could be quite chilling on some of the darker maps), a simple deathmatch mode, and a one-on-one infected vs lone ranger mode.