Okay, so it obviously wouldn't be a GTA game, but if they make RDR, they could make something like this.
Every time I see a show on Rome on the History Channel, or watch a movie like Gladiator, I think how awesome it might be to have an open-world game where you play as say, a Roman soldier, and you would have Ancient Rome, small towns in the surrounding area, and maybe be able to take to the Mediterranean and get to Carthage, Eqypt, etc.
I mean, I guess the subject has been beaten to death, but I just think it would be a cool game, if done right, and R* usually does things right. Think RDR minus guns, plus swords. Horse would still be the main mode of transportation.
My first thought would be the lack of guns/shooting would be very missed, however I think the ability to run into a huge battle with sword in hand would be a nice alternative. I don't mean you versus 5 opponents either. I mean being a member of a Roman legion, with hundreds of fellow soldiers, going into battle versus hundreds of enemies. I think it would be epic.
Off the top of my head, the plot to the game could be very Gladiator-esque. Start as a gladiator, battle to win your freedom, then you're in the Roman army. Earn your way to General, eventually becoming a Roman politician and one day Emperor. Decisions made as politicial/Emperor could effect the happiness and behavior of citizens, while being a terrible General could cause your armies to turn on you.
This is nothing more than a pipe-dream really, but I'm interested to see who would like a game like this, and what ideas some of you might have for it.