- A flyable jet, like the lears at the Airport. 
- An attack helicopter/airplane. (like the Apache in ALL HANDS ON DECK, or an F-16 that fires missiles) 
- Residental neighborhoods. 
- The ability to steal things from businesses/homes. 
- The ability to sell things. 
- The ability to buy/steal things to customize a vehicle/place. 
- Mini-Games (like the annoying arcade game in the Taxi garage...but playable) 
- Legitimate car/boat/aircraft dealership. 
- Character customization. 
- Variable destination Taxis. 
- More control over your gang. 
- Diplomacy with other NPCs (Police Chief/Mayor/Other gang leader/etc.) A cellphone would be a good interface for this. 
- Swimming. 
- Gambling. 
- Many more Interior spaces. 
- Neighboring small cities/towns that can be visited. 
- More control over business assets. 
- Better police AI.  (Why surrender, it dosen't get you anything?) 
- Diffrent military services. 
- Better pedestrian/driver AI. 
- Ability to attach trailers to certain vehicles.  (Boat trailer, car trailer, plane stairs at escobar, etc.) 
- More realistic carnage. 
- Higher detailed people models. 
- A brothel, lol. 
-More, I'm sure, but that's all I have for now.