A realistic weather system really is something games have never used before, so I'd love to see that come the next installment in a few years. And with the next gen tech, its doable. I'd also like to see at least a 1/4th of the buildings be accessible. Definitely keep the full character customization along with the cars, and purchaseable properties. I love that aspect in games.
I like the stocks but you can only really make money off of them during the assassination missions, I'd love to see tht fluctuate like the real market. Also, I can already tell online is going to be unlike anything on the market, and will probably be an instant hit. Map needs to be way larger. It's perfect on V, but i want it to take me good while to get from one end of the map to the other.