I think what GTA needs is the use of artificial intelligence for the NPCs. There should be some random mini-missions with them. Like helping someone get a cat out of a tree, cleaning the roof of some house, etc.
I also think that the online mode should be 100% in the game. Even in the missions. Where other players would help you and everything. In my opinion, single-player and multiplayer shouldn’t be two distinct modes.
Another cool thing would be to have an item and inventory system like they do in FiveM, where you would have many rare things that others don’t. This would even give players the opportunity to sell items for real money, just like it’s done in Counter-Strike: Source.