This could be kind of a stretch, but you might want to check out Assassin's Creed. It quite different from the R* games, but its got an open world, sandbox-y feel to it. You can free run all over the city and kill guards in spectacularly brutal ways, manage assassins, factions, find collectables, the whole lot. And the actual gameplay (free-running, combat, etc.) is buttery smooth. It's a pretty amazing game all around.