The sheer amount of content in 'GTA V' can actually be a bit overwhelming. There are hundreds of different vehicles spanning land, sea and air spread across miles of city and countryside. Populating those miles are a seemingly endless amount of AI characters going about their day-to-day business.
There is such a huge range of options about what to do next, that there are times that it can feel like 'GTA V' has virtually everything in it. From time to time though, something stands out as missing. Some of the things missing are just items in the game that would fit given all the other items, such as a specific vehicle. These could very easily be added later as downloadable content. Other things missing are more of game mechanics and would have to be implemented at a game-engine level, meaning it is almost impossible they could be added to the game at this point.
This list is obviously subjective, so please feel free to comment and add anything else that should have made its way into 'GTA V.'