16. Seasons
This is a weird one, but one that I think could add so many more variables to the gameplay that you'd almost never have the same experience twice: a seasonal year with a functioning calendar. Sure, GTA V is set in Los Santos. If it stays true to the seasons of Los Angeles, itll be warm all the time and rain in December. But since we know that there will be more explorable terrain outside of Los Santos, we can assume that the geography will be similar to Southern California. That means that there's mountains, snow, rain, and all the temperature extremes that come in Winter and Summer. Missions requiring chases or high speeds would be made significantly more difficult on snow. Aside from the obvious physical differences, there are visual and ethereal changes that would occur. Further away from the city, leaves might change in the fall. Skies would be clear and blue in the summer, cloudy in the winter. The sun would set sooner or later in the day, depending on what time of year it was. Missions would have an entirely different feel to them if they were played in cold weather instead of warm weather, much as they already do in existing GTA games when played in day or night, dry weather or rain. It's not a vitally important element to the game, but it's that extra attention to detail for which Rockstar is known.