Enterable buildings, most places you couldn't enter, just your own safe houses and a couple of other places.
More realism, blood and gore it is always going to be an 18 rated game so why not have it manhunt style.
When you run over someone they don't just flip over flat, they roll onto the car and fall off the side.
More realism on crowds, when you kill someone like 4 people run over and stand there then walk off, better if it was like them shouting things like "call the police" or something.
Judging from the graphics of the trailer I couldn't complain at all.
Better audio on certain parts again such as the cars etc. More variations of civilians, not like 10 variable civilians so you see like 10 of the same people going down the road, takes more time yeah but its 2007 heh.
More variety of cars, bikes etc.
Upgradable equipment, such as silencer for your assault rifle.
More stores etc, places to buy weapons etc and also weapon dealers where you can buy cheaper but say their would be more of a risk like you could get screwed over.
Dunno if anyone remembers but in GTA/2 you could hijack a car and it would come up saying something like "Nice try theres a bomb in here have fun getting out" on expensive cars, that would be pretty cool.
Larger map, long highways, lots of rural areas like GTA SA did.
Factions, the ability to join gangs or even create gangs.
Protection rackets, alike the godfather.
Off the top of my head for now.