Ok then.. here's my suggestions.
#1: A Player Auction: To auction off stuff that players don't need. House gets a %.
...and no I'm not talking about a trade server. An actual auction. bid or buy.
*** Now for some fun stuff..
#2: A Boat: like the cars. only on Water. uses gas like the cars. repair and all that.
#3: For the fun of it.. A helicopter or jet pack. uses gas. it runs out real quick tho. O_O Like the zombie repellent. but not as fast.
#4: A rocket launcher or grenade launcher.. uses frag grenades. difficult to find the launcher tho. yeah.
Ok. How does any of that sound to you.. eh?
It's All good here though... keep on working on the game. nice!