I want the ability to get away from the cops without just simply having to exit the wanted radius. There's been plenty of times I've gotten myself hidden in a spot where in real life they'd be confused and possibly walk right passed me without seeing me. But, in the game they always know how to beeline to where ever you are as long as you're in the radius. The ability to lose them by hiding would be great.
I agree with the poster who asked for a more realistic pursuit by the police as well. When you commit a crime or have a wanted level, it should take someone actually notifying the police (like in Red Dead Redemption) to get the cops after you. Also, I hate it when you destroy the whole police force chasing you including the helicopter(s), and more just spawn right behind you. If you kill them all you should have the ability to flee the scene and get away, rather than more cops/helicopters just magically appearing, always knowing exactly where you are as if your character has a tracking device on him at all times. I'd love to be able to slide under cars or hide in buses or buildings to confuse them. The ability to impersonate an officer would be sweet as well.
I like the relationships, but wish they'd make raising the levels on them more interesting and less redundant. In GTA4, I must've taken the same girl out on the same date with the same conclusion 20 times. Boring.
I would love it if the buildings could be damaged. Driving a car into a brick wall or firing a missile into a building doesn't do anything to it. Would love to see some destruction that would last at least until your character dies or gets arrested. Knocking buildings down completely would be lovely.
The ability to grow facial hair and shave. Not just going to a barber like in San Andreas.
I'd love it if they upgraded the hand to hand fighting with people heavily. The ability to pick them up and swing them into things and breaking joints. Throwing them through glass and seeing multiple cut wounds. More variety with weapons like stabbing/slicing people in specific parts of the body and seeing different reactions.
Just a few ideas.