got 2 more:
1) race
it always bottered me that u cannot do proper race in the game, so mabey there could be game only for races. people could choose a race track (from sunshine autos where spawn point would be (in gtavc)) and they will line up there with others who have chosen this track, countdown begins and they do check point race, with traffic on of course. for winning the races they could get money for which they could buy better cars or something, but same cars would be more fair. also cops chasing the racers could be fun.
2) many people would like game to be interactive, so mabey things like sitting in the bars, buying drinks, money, and so on could be added. just so people can role play