It needs to something new and fresh because as it is GTA5 is as perfect as it comes, Vegas-esque seems about right because it would allow them to add in a massive gambling aspect which I don't think the series has really touched upon.
I'm not sure what's up with the comment above about the parody, GTA5 was flawless, I haven't even touched online, but it's there for people who want it, as a single player, it was fantastic beyond belief though, and I can still throw it in and enjoy the hell out of it. More so than 4, which itself only gets flack because well it's not quite SA or 5.
But as I said, it's gotta move to gambling....