I'd like to see the city finished off. There are loads of shops that are versions of existing places according to the facade but cannot be entered. Simply giving those places carbon copies of the insides of their other versions would be a nice start. So we'd be able to watch movies at all of the cinemas not just the three chosen by the devs and able to buy clothing at all clothes shops as well as rob all of the stores. I know a lot of things can be ordered online, but having a few places in the world where we can buy bikes, cars and the like instead of just having them available for missions would again go a step towards fleshing the city out. Finally add in the food places from the last game so we can enter Burger Shots and the like, and buy hot dogs on the street rather than looking for first aid kits hidden around the city. Bam, living city with a lot more perceived depth than it had before.
Also, a single player heist generator would be a nice addition. Available from a new property to buy you'd have a whiteboard to pick a place to rob. If it were one of the established heists then the other version of it would be available to carry out (and then only once the story mission has been passed) before being locked. If it is a new place then you get a generic robbery with hostage management situation, and have to fight and drive your way past the cops to a set extraction point.
Heists would be set up by choosing a place to rob, choosing crew and marking an extraction point (driving into a truck to hide vehicles, making it to a boat on the coast, get to the helipad, etc). Extraction points would lower the wanted level by a few stars (more difficult extractions lowering them by more) and the mission would complete when the cops were lost and you made your way back to the original property. Give a week to launder the take and up skills of heist crew by 5% and we got a basic system.