I personally think setting the game in present times would be a terrible decision. It would be GTA V all over again with repetitive parodies about social media and technology.   The best time frame to set the game in would be the 70's IMO. It hasn't been done before, a lot happened during that decade so there's a lot of gameplay/theme possibilities and stylistically it would look awesome.   Another modern day GTA game would be a horrendous decision.