They should next gta in vice city but in two time periods, the 80s and the present day
When I read that, I immediately thought: the protagonist, a man in his 50's living in Vice City, constantly reminisces on his life back in the 80's (we see, and get to take part in, flashbacks), until he decides to get back into the life of crime. From then on, we get to play as him in his 50's and do missions as such, but occasionally he will reminisce to his life of crime in the 80's and try re-creating events that happened back then, or retrying past failures. And then, once the main story is complete (or possibly even before it's complete), the player can switch between the man in his 30's (the 1980's) and in his 50's (present day): the camera would go up, the city would change, and the camera would zoom back in to the player - pretty much like the character switching in GTA V.
That's just a quick idea that I had, and it would definitely work if the right crew were working on it. I'm not sure if it would be the best for a GTA game, but I quite like the idea, if I say so myself.