I'm to lazy to read every post here, so sorry if I suggest something someone else already said.
Scuba Gear, for missions where your under water for a long period of time. And be able to lose cops under water. There's jetpacks and parchutes, why not scuba gear.
Buy or steal walkmans to listen to music while not in a vechile.
But the two player mode should be improved alot. Like if you go too far apart, instead of going back to one player, have the screen split, so you don't have to stay together. And and when you come back together, have the option of joining screens. And the passenger can shoot in all directions, not just to the sides and ahead. Also, be able to get into one vehicle and second player get into another one And More then one person can get into big airplanes like that huge jumbo jet at the third airport. Same thing with boats.
And have more options at the mod shop, usually there's only two bumpers, side skirts or spoilers to chose from. And more decals. Maybe design your own decal, or tattoo, or gang signature.
Also, have a more open ended game play, like GTA 1 and 2. Be able to chose which gang to join. You'll start at a low rank and work your way up to a mob boss or something.
Or you can have a carma meter thingy. The more bad stuff you do, the more you are wanted. Or feared by peds. But to stay good, you can hitchhike, get taxi's or ride trains or planes. Maybe have the choice to become a gang member or join the police force.
To keep your carma up, you can buy cars. Or have the ability to hotwire cars. The better you are at hotwiring, the faster you can do it. Just make it more challenging to steal cars from dealer ships.
Some one mention about building your own house. For your house to can buy stuff, like refridgetors ( to eat at your house), and tvs and all that stuff. Or you can steal them, from the store or other people houses.
Vehicle I'll like to see
- Submarines
- and a subsystem (Like GTA:3) only you have the choice to ride it or steal it.
- skateboards
- unicycles (for a laugh)
-
weapons
- more silenced submachine guns
- fibre wire (to strangle people)
- turret guns on back of some vehicle ( maybe the army hummer, have some one drive and get someone in the back to shoot)
I'm just going to stop here, cause my list of idea's is just going to get bigger and there's no end in sight. There's so much they can do with the game.