The only thing I would change about GTA V, is the map. The map is huge and it's great BUT, the biggest drawback is the fact that I can't recognize most of the places. I have played San Andreas for a very long time, for about 5 years non-stop, and on and off every now and then. I know the San Andreas map by heart, I know where everything is. I can imagine going from Los Santos to Las Venturas, to San Fierro and back to Los Santos accurately without even playing the game. It stuck into my mind and I know the map better than I know my hometown. However, GTA V's map wasn't very recognizable to me and they have changed A LOT about it, and that's something that I don't like. I would try to make it resemble San Andreas a little bit more. They even changed the Groove Street, which was VERY important in San Andreas, to the point where you can barely find it, and once you do, many things are not like they were in San Andreas. For example, the CJ's house, YOUR house from GTA: SA is completely different.
Another thing I would change about the map is add in San Fierro and Las Venturas. I can't explain how happy that would make me, since I've played San Andreas for quite a long portion of my life, and that would be like being reborn. Obviously, I doubt that's ever going to happen, and considering the game is 50 GB even without that.