I've always thought how well an open world Back to the Future game could be. Like you could get in the delorean, drive around Hill Valley and then time time travel to mess with the future or past. And in the case of messing with the past, then the future timelines would update based on actions you make.
Obviously it's get complicated really fast, and somehow you'd have to have some mechanic to make it so you can't interact with your past self. Maybe some sort of reset timeline mechanic would be needed if there were too many versions of yourself in a certain time?