More fluid physics, niko seemed really clunky and awkward to control. Perhaps an auto climb feature for walls and such like in LA Noire. I want to be able to interact with the environment more than you could in IV. Hell, even basic free running would be cool.
Fun cheats
Zombies
Ramps/stairs etc you can actually jump. I dont wanna go flying off the bike when I hit the steps, I wanna fly over them, jump off mid-air, and watch my guy's head hit the pavement.
Enough to do outside the story. People play the story for 30 hours then spend the rest of their time fucking about. There's no point pouring all the development time into the story when people will play it once or twice and then jump to killing hookers and doing stunts