Firstly let me say that I think this is the best GTA game that has ever been made. However, if they had implemented a few minor changes to this game, it would be perfect. A couple of changes that would have been so easy to introduce, that its actually mind boggling that they didn't.
1. Mission Outfits - How can they tease us with awesome mission outfits such as the full body kevlar suit that it used during one of the heist missions, then not let us use it in free roam. It would provide people with so much fun, just putting on this bad boy a wreaking havoc with a mini gun in downtown Los Santos or going to the army base. It would be so easy for them just to have a Mission Outfit section in each players wardrobe. It would also allow role playing with the police outfits, etc.
2. Saving Cars - Rather then the outdated and unreliable method of parking you car in the garage and praying that it remains there, and then not being able to drive it on missions as it will just disappear. Each character should be able to get in a car and press a button/s that saves the car to that character. Obviously there would be a limit to the number of cars that the player can save, but this would be a much more reliable method of keeping the car that you want. And just stealing cars that you don't care about.
3. Replaying Heists - I know that there is the ability to replay a mission in the game now. But at the moment it is pretty pointless. Once a player has reached 100%, you should have the option of replaying the entire heist (including setup), and the money that is earned actually goes to the characters. It also gives the ability to do the other method for the heist. And it keeps people playing the same story line for longer.
4. Purchasable Safe-houses - I know this has been done to death, so I won't go to far into this. But there should definitely be the option to purchase more safe houses. It would be cool if you could purchase houses in the different areas, such as Chumash, Vinewood Hills, Downtown Los Santos, Mt Chilliad, Alemo Sea, etc. It would make the money earned in game mean a little more. Its all well and good, purchasing businesses that earn more money. But whats the point of earning more money if you have nothing to use that money on.