the biggest most glaring one is expanding the single player experience
GTA 4 had 3 single player DLC expansions. Now, the online was still early in its infancy, but they put the single player experience at the forefront. I'm 36 now. My online days are coming to an end. I dont have the response times with my hands/fingers as I did when COD 4 was out, and my time is not as abundant either. I will ALWAYS place single player as the priority moving forward.
I think they missed another huge opportunity not doing ANYTHING with the casino and the race track that were in the game.
I think they heist's were a bit misleading from the dev videos they dropped showcasing how they would work. All of it was still very much linear with defined outcomes that all lead to the same end result, that, iirc, had no real consequences on the rest of the game. Sure, you might've heard something on the radio after a heist, but there were no real stakes when engaging in robberies. They just became more increasingly difficult as the game advanced. There was no real planning the heists. It was "pick route A or pick route B".
now, i say all of this as a "gamer" but i do realize that with a game of this magnitude, there is no way to make these things anymore dynamic than they were presented as you would have to test hundreds of different outcomes to see ow they all panned out to give some more variety in how you came to the inevitable conclusion.