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 **** guards in spectacularly ****** 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.