one of the things i would like to see in gta4 is, when you blow up a car, it would stay there untill you drug it off, or the city would send towtrucks out, but how many times ive blown up like 50 cars, and when you turn around your pile of cars is gone, thats annoying.
i would also like to see more interaction with the enviroment, like having animals for hunting ( like big game hunters) inside the game, or the ability to grow pot in your backyard, and actually care for the plants(like in high-grow)
and also, more like real life, if you launce a rocket at a house, it doesnt bouce off, i wish the house would burn down, then you could clear away the debis and manually build a new house (ala the sims) or the game would build it back on its own, or you could burn the whole city down, and just go nuts killing and looting.
and i think in the new game, they should include the cheat codes with the game, because cheating is fun