My vote goes completely to a western setting; simply not enough games that tackle that particular setting/genre.
Dinosaurs; post apocalyptic; sci-fi; these are all very well tread (with perhaps the exception of dinosaurs) settings in videogames, and let's be honest, other developers will re-visit them again... and again... and again.
A western setting could mean hunting serves an actual purpose. It would also allow for first person horse riding; payday inspired robberies; payote trip alternate reality shenanigans; could even try and do something regards fps duels and nick some nemesis stuff from Shadow of Mordor. I also reckon a decent story could be crafted within that setting as well. Varied landscapes possible as well.
The other direction they could take which isn't mentioned is to try and outdo GTA and take it to the city and surrounding area, only make it you are an undercover cop. Guess that would be a big ask though and tone would be difficult to get right, along with a city-scape perhaps limiting the kind of nonsense one could get up to.
First post on GAF, so go easy folks