Buying Different guns for a bigger chance of a successful wack attempt or for self protection, or maybe assigning them to Bodyguards 

like selecting different kinds of pistols, revolvers and of course... every mobster's favorite.. the "Tommy Gun" 

well obviously everybody would buy a tommy gun, but! each gun would have its different maintenance cost and different ammo costs

now wacking would not be free, it would cost you Ammo and for your gun to be effective you would have to maintain its durability by maintenance.

there it is, feel free to add ideas and I hope you like my suggestion. :)

NOTE: The Guns aren't for free, You will have to Buy it and some are only available for certain ranks ***

I like it this way because it's unique when compared to other mafia online games, which mostly have gun, protection and bullet factory.