Either Create-a-gang, or at least have the ability to hire a little backup from a gang you're in good graces with.
Ability to CHOOSE colors for your car at the paint shop, and possibly buy cars (delivered to chosen hideout garage each time it empties) from gangs you're in good graces with.
Ability to make amends to gangs.. Stupid Maria.. >_<
Motorcycles, of course.. those would kick ass.
Have the guy swim, for cryin' out loud..
Police not running over a hundred citizens of Vice City to get to you..
Police that arrest other people as well..
Random gang wars that you can instagate/participate in. These were depressingly seldom in GTA3.
Timed import garages ala Gone in 60 Seconds.
Car customization.. bulletproof glass, nitrous, hydrolics.
Respect meter. One that's as badass as your character by the end of GTA3 shouldn't have piddly diablos f****n with him, y'know?
Better first person car view.. I felt strapped to the bumper of whatever car I drove in that view, not like I was in the driver seat.
Ray.. gotta have Ray in Vice City.
Go into buildings.
Ability to commit crimes other than just killing people and those allowed by game plot. IE go into a convenience store and rob it, cause a riot, that kinda cool stuff.