I think there should be schools/colleges with kids but you can't do anything to them.
Better physics.
Access the whole GTA world-San Andreas, Liberty, Vice, Carcer(maybe), Bagota, etc...
More cars
More Missions-Also Harder
Buy stuff for your place. (I.E. jack a Mule, go to Best Buy and stock the mule with a computer, plasma TV, etc)
Gettting gas is a cool idea
Everything else the other Chris and Spaz said.