I know it's kind of a controversial addition to a lot of people, and it's kind of dumb, but I have been wanting a fuel system for vehicles in the GTA series for YEARS. It adds just a little bit more realism/immersion that could add to all of the other things to make this one of the most immersive open world sandbox games of all time.
GTA 4 is definitely my favorite in the series. Even the DLCs were amazing. I really liked The Lost And the Damned DLC since it added a death metal soundtrack! I'm hoping we get some good heavier music in GTA 6 as well, since the real-life Florida region has a pretty big thrash metal community!