Just a small wishlist for me.
I would like them to continue with the 3 Protagonist story, have one be a female, and the white guy to be either from Ireland or England, and definitely, have the black guy break stereotype (well, I guess he'd still be a criminal), make him the hacker and ex-military, but in addition to that, they'd each have their own story lines along with the primary story line, where you would be able to control other characters that are allies of the main characters.
I really hope they go with a multi-city approach, either where it's on large map (like the triangle of San Andreas to Las Venturas to San Fiero) or where you'd have large individual map and you have to take a plane/train/bus to travel between them.
With regards to the online, add some type of skills or payday like classes. Some type of specialization, think Ocean's Eleven. You'd have classes/skills that would be like Mastermind, Muscle, Hacker, Driver, Pilot, Shooter, Chameleon, etc... Heists would be heavily random (location, target, security etc...), depending on classes and things you do during the set up (hack computers, scope out the area, bribe people) would determine how much information your characters have and you're options for how to do the heist, making it easier to do a "Perfect Heist". You'd also have to work up to more profitable jobs.
Money in online should also be more fluid, more come and go. The higher end homes/apartments should have a higher up keep, the higher end cars should have higher insurance costs compared to the lower, the bank account should gain interest and the stock market should work, even if it has to work in a very different way. This way those who save money get something for saving money and those who splurge have more of an upkeep.
The party system needs to be improved, too many times I started a mission with my friends from the same lobby, only to finish and each wind up in different lobbies.
I think you should also build up a rap sheet, as you get caught seen commit crimes. It would be a fun challenge to try things and to remain completely unseen. A larger rap sheet would mean the cops would be more aggressive, civilians might be more aggressive, you might get bounty hunters on you if you become infamous, and if you have a hard time being stealthy, you could bribe a politician or police chief or IAA or FIB agents to scrub your record.
I think each of the areas of crime should become far more deep. For example, when starting out, you could only bring the cheap cars to a chop shop, but as you gain trust, you could then start bringing them higher end, and eventually super cars. The missions should also become more malleable. Think "Gone in 60 seconds", you might be given a task to steal dozens of cars, how you go about it is up to you. You could break into the dmv and steal info on the cars, or hack into the system, sit on the cars to either steal them when no one is around or drag the owner out and jack his ride, you could do it over a several in game day period or blitz kreig it. And the actual condition of the cars would effect the end pay out.
More free roam missions. Having to go into mission lobbies is not only clunky it break immersion.
Better racing controls. Give the ability to drive either manual or automatic, allow us to turn on or off a driving hud.
Last but not least, correct in the game menus. These damn cars don't move nearly as fast as they claim to.