Ok, here's a list:
1) More missions.
2) More than 1 person on a bus, 2 people in a taxi, etc.
3) More garage room would be nice. I say do away with the car limit, just let us cram as many cars as we can into the garage.
4) Being able to sell any car you want would be nice too, like you could take it to the docks and a ship would pick it up and give you money, for any car in the game (some would be worth more than others, of course).
5) A bigger area and more secrets to find
6) A weapon between Shotgun and Uzi would be nice, something that would kill with 1 or 2 shots, but still be quick. That shotgun takes forever to reload, and you can't run with it either, while the Uzi is light and reloads quickly, but is very weak compared to the shotgun.
7) The ability to fly planes that aren't missing important parts (wings), also helicopters, and it would be nice if the game would remember damage you dealt to things in the city, and have workers come to repair things.
8) multiplayer mode. Wtf were they thinking when they didn't make this game multiplayer?
that's about it for now