Rockstar have talked about wanting to expand GTA:Online to possible other locations, and I think Las Venturas should be added for both SP and MP.
Not only could you do heists on Casinos etc, but they could also add gambling activities similar to Red Dead Redemption. The mechanics are already there, just bring them over into GTA V and allow us to gamble both in SP and MP. They could also have shows and acts etc that you could watch, similar to the real Las Vegas.
They could even reuse assets from Red Dead Redemption for the nearby canyons etc, I'd be cool with that. Rockstar could weave a good story around it to include Casino heists and I think Las Venturas could be a great place for some really colourful, crazy characters.
It needs to be done in my opinion.