Release a handful of city-specific editions, both national and international.
Conceive of a better use of the money you make. Allow for the purchase of souped-up cars, boats, and aircraft, exclusive weapons, better bribes, and home improvement.
Create a better system for organizing and displaying on-hand weapons, Some weapons require you to unload others, and it’s not always clear what you’re giving up.
Allow manual aim on all the weapons, not just the rifles. It’s infuriating to be armed with just a pistol, trying to save ammo, and being unable to pop off headshots, or shoot someone inside a car.
Improve the aiming of the hand grenades. Other games incorporate some sort of target you can move control so you can see where your grendae will go. In GTA all you can do is throw it in the direction you’re facing and hope you held the button in long enough so you don’t blow yourself up.
Allow the character to strafe. You can’t lean out from behind corners and shoot like you could in Syphon Filter for example. Bugs me.
I would love to see a 1990’s or present time set in Las Vegas. 
What about the soundtrack?
Modern times:
Nirvana, Come as you are or Smells like Teen Spirit
Hmm. Can’t think of anything else. As per LifeIllustrator’s suggestion, if it’s the 90s, why not go with a fake band for whatever decade the game is set in, like Lovefist? Perhaps a rap or grunge band?
Or just a general future setting.
How about a Hunter Mode, where there’s one character on the map that doesn’t show up on the radar, but stands out, due to his supermaniacal activities. He’s got all the weapons, a beefed up health meter, and tries like hell to keep you from killing him, escaping when convenient and attacking when trapped. Your job is to find him (which is rare), and eliminate him. This could also be used as a format for 2-player scenarios.
Realistic car damage. Difficulty aside, I’d rather my Stinger become a crumpled heap when I broadside an unsuspecting Idaho at 60 MPH, rather than spin like a top and drive away with only a bended fender and a flapping hood.
Sub-level areas like the train stations in GTA3. Tunnels are cool. They make great places to hold up and hide behind piles of car rubble, equipped with your own personal arsenal. Too bad the wrecks disappear after a while… something that memory and processor speed will eventually handle.
Fully-operational construction vehicles: plows, backhoes, cranes…DESTRUCTION!
The best thing about a game like this is, every time you’re sitting there playing it, you can find several little things that can and will be improved sometime, since a game like this simulates real environments with so many variables. Long live GTA.
^^ I definetely want construction vehicles.
But I also want a new arrest system. How about if a cop touches you, the controller starts to vibirate and you have to button mash your way to freedom? And if you manage to escape being handcuffed, the next time you’re caught, you have to button-mash even harder?
Its a better system than now where a cop just opens your car door and you’re busted.
Imagine the fun you could have with multiplayer.
Get a phone call from a drug lord requesting you take out the rival drug lord. You set up a “deal” with the rival, getting there in your Sentinel XS. Your character shows he has no weapons, this deal will go down without anything bad having to happen. “Hello, Mr Mercitti-”, the drug lord cut off by your friend popping up out of the trunk and filling him with lead.
Have it so that you’re driving while a friend plays the gunner, for either the humvee, the police boat or the FBI plane ala those humvee things in halo.
Great ideas, guys.
If you haven’t already heard, a multiplayer mod is in the alpha stages. The team claims that although multiplayer was ultimately dropped by Rockstar, the multiplayer code exists in the original game. Team deathmatch is the only play mode currently offered in the mod.