Heres a complete list of what i think should be planned for the game:

camera - everything third-person

level - various sections of chicago(not all of chicago)

people - you,pedestrians(walk from point a to point b),employees at shops...

controls - arrowkeys = movement,mouseclick = shoot,spacebar = pause menu,shiftkey = zoom(on guns able to zoom),enterkey to enter vehicles.

vehicles - cars,buses,taxi's...

time - modern

buildings - airports,houses,commercial buildings,landmarks(most will be interior/exterior unless there is no reason to add interior)

loading - probably in sections to maintain fps

items - guns,health,money,cds(possibly you could have a game radio, and could pick up new music to play?)

audio - radio(possibly in cars),sound(gunshot), background music,voice

guns - Unarmed,AK47,Uzi,A15,M16,MP5,Shotgun,Sniper,Pistol