i got some great ideas....
you could make this like a GTA game but with more squares
you should add more characters in the game e.g different mafia members, dealers and police.(you can make it that once you kill a certain amount of people, cops come after YOU instead of your boss)
you should make extra challenges such as. kill someone (hitman) or deliver something safely ( like money or drugs).
you should also make cars (nothing complicated or that has to look good just something you can ride on and shoot out of) for getaways and later on ( in another version) you could add more veichles e.g motorbikes, boats
add stores so you can upgrade on guns (dont bother on ammo), so you just start with a hand gun but you later on upgrade (this adds more of an aim to the game instead of just repeating levels) you get the money when you kill people (points)
for weapons i recommend more explosives like basookas, granede launchers and sticky bombs. also a big badass machine gun (like the one on predator XD) and for fun a retractable harpoon XD. throwing knives would be awesome too.
a mini map would be cool that shows you were to go. and like you said more buildings would be awesome.
you could add a story line (campaign mode) like you work for a gangster as a body guard and you got to protect him from police, mafias or hitmans. and you get higher and higher at in the mafia etc etc.
also it would be cool to have multiplayer. you can make different game types like:
Track and kill: each player has a handgun, the the aim of the game is to kill the other players but the trick is, that the game doesnt tell you if its a player or the computer, so you have to tell by their movements or their actions (running, shooting etc) once you kill some one that is a player you upgrade on guns when you kill 3 bots you downgrade your gun (if you downgrade completely you only have a knife).
mafia wars: the game split the players in teams of 2 or 3 and each one has to kill the other teams boss (chosen by computer) while you protect yours. when you die you respawn at spawn point and you start with the gun of choice (unlimited ammo).
protect the target: a player is chosen to protect a briefcase for each team. the aim is to steal the enemy briefcase and deliver it to your spawn point. while also protect the wielder of the briefcase from your team.
also a sandbox mode, choose and edit the type of game you want to play, amount of assassin bots (or zombies) and the time of the game till it ends
later on another version you can add more sites e.g parks, ports
i suggest that you make the graphics the same or just simple with rounded squares. nothing realistic or else it would be too big to play for most people.
this would make it a great game for people that want more gangster an mafia games (there aren't that many) and for people with old computers that cant run huge games...
i hope you like my ideas... XD