If they're going to bring Las Venturas into the mix, they should do it for GTA V, while we're still working with a heist theme. As I've said before, Las Vegas is just a bunch of casinos, a suburb, and empty desert. Las Venturas cannot carry a game on its own. San Fierro can.
Las Venturas DLC for V, and GTA VI set in San Fierro.
And I do not want another heist-themed GTA, unless there are a lot more heists than V had, a lot more money floating around, and a lot of things to spend it on. I can go on for pages and pages about this, however, and I forbid a GTA VI wishlist until V has been out for at least a year.