I'm still playing GTA4 Complete Edition, so I've been thinking of things I'd like to see in future versions (maybe not possible for #5).
I want to have a button to push to talk to people on the street like in an RPG. Even if most of them had nothing important to say it would be cool.
Dating and friendships should be like Mass Effect, with dialog choices and decisions to make that affect whether they like you or not.
If people are going to be calling you all the time to do stuff with them, they need more activities. Pool and bowling are cool, but you can only do them so many times before it becomes a chore. How about mini golf? Batting cage?
Bring back some stuff from SA, like riding bicycles, and working out or eating a lot of junk food to change the way your character looks.
There's more, but can't think of them right now lol.
"Machete don't text"