2. Side-mission Campaigns: Rival Gangs
This had been a focus in GTA: San Andreas. Remember those colour coded map points where you had to clear an area of gang members before converting it over? It was essentially just a mindless killing spree. To reintroduce this in GTA V but also involve it with a narrative driven power struggle too could be more involving for the player. I can see having 3-4 gangs to choose from, having missions of assassination, stealing, and imposture, and each string of missions with its own ending and/or reward for completing them. Choosing a particular gang could also have all sorts of repercussions on the world, giving access to different weapons, houses and/or clothes for the main character.