Allow the character to swim in the water for a short time. This would allow you the chance to swim back to land if you bail out in the water.
Can't tell you how many times I've wanted this recently. I've been goofing around in VC, and I hate how I can drown in chest-high water. Swimming can't be that hard to implement, since they already have floating cars. You could use almost the exact same code for at least character dog-paddling. If the game designers didn't want to make it dead easy, you could have to practice your swimming to get good at it, like how Tommy gets skinnier when you sprint around whenever possible. Or maybe you have to pay a bunch of money or complete a side mission to learn to scuba dive or something. This also opens up the VERY cool possiblity of that big old shark coming along and munching on you while you try and swim to shore. Wouldn't that be a awesome way to die! Or the dolphin can come buy and either playfully jump over and around you, or help you to shore.
Better AI in general, especially for the police. Make the police have to beat you to the floor to bust you, instead of just opening your car door.
On a related note - how about if someone OTHER than you does something bad, the police go after them? I mean, there's that "randomly selected guy turns into a thief and cop chases him" thing - hit the guy with your fist and you get a $50 "good citizen" bonus (but don't use weapons or you'll get a star, and the cop will be right there to bust you!).
But if a gang war breaks out, the cops just stand there, or even walk right by! It breaks the reality of the game a bit. The city is so alive and realistic in most other ways, this is really glaring to me.
One thing I've been doing, just for something to do since I've finished all the story missions in VC, is to hang around the road at the north side of the North Point mall, and watch the blue uniform security guys and the mall preppies get in gunfights with each other. When a mall preppie goes down, there's usually five to eight bundles of cash on the ground right around him. So I go grab the cash, dodging any gunfire. The ambulance comes by, resurrects the security and/or preppies. Rinse, lather, repeat.
And yet no cop ever notices these huge bloody gunfights that often cut down innocent bystanders driving around the mall. Even when they're standing right there! Same with other cars that ram into cop cars. How come the cops don't go after them? It doesn't seem like this would be much harder than what the random people on the street AI does now.
Non-lethal weapons might be cool. Teargas was done in VC, but I never used it. How about a tazer? Tazering a cop only gets you one star, and he falls over stunned for, say, 10 seconds - just long enough for you to get away if you're smart and there's no other police around. Maybe some extra-expensive tranq gun ammo for the sniper rifles? Missions that require you to capture, but not kill, someone?
The katana was way cool, but I wished you could use it for more different kinds of swings. Since I aspire to know how to use a katana IRL, I'm probably a little more fanatic than most people. Might also be kinda cool to be able to train your character in some martial art or another. Would allow you to hit harder and maybe block, dodge or catch attacks depending on whether you studied, say, karate, kung fu, or aikido.
As far as vehicles go, VC was damn good. Helicopters and motorcycles, both implemented quite well. Big fat hairy kudos to Rockstar for these two. Damn they are fun, fun, fun!
I think the only vehicle I would want to use that hasn't been around is a wheeled Dodo (probably fly pretty much like the skimmer, but you could use it on land) and that Lear Jet you can see at the airport. Flying that would just be too cool. There are still several air vehicles that Rockstar could do. Ultralights, hang-gliders/parasails, jetpacks, etc. I'd like there to not be a height limitation on the air vehicles that is lower than the top of any building. Players really should be able to get to the top of every building. How come I couldn't attempt to board the blimp in VC? Weak, man!
It would be nice to be able to shoot from helicopters without having to engage cinema camera. Or shoot something other than your uzi from inside a car, like say your sniper rifle. That's pretty minor though.
GTA3/VC was pretty good about letting you modify the environment, what with knocking down streetlights, etc. It would be cool to be able to blow up walls or take down structures too. Hard to implement this one well, though. It would probably have to be restricted to certain places only, because letting you take down whole buildings would be really hard to keep track of. Not to mention the amount of explosive you'd have to plant to take down a moderate sized building. Still be fun to, say, blow up the El tracks in Liberty City!
I believe that rumors are that they're already working on a MMORPG very much based on Vice City. In the mean time, I wish the http://www.multitheftauto.com guys would hurry up and make GTA3 multiplayer...
And I totally agree with whoever said a speedo for the cars!
I just went and check the MultiTheftAuto site for the first time in a couple of months. They've apparently gotten Vice City hacked sufficiently to do 4 player! There's a (19 MB zipped!!) movie if you want it here: