They wouldn't do what they'd need to do in order for heists to be worth the wait.
Las Venturas cannot stand on its own as the setting for a future GTA. There isn't enough there. They need to bring in LV for story mode and online, with an entirely new singleplayer story revolving around hitting major casinos, and the three protagonists acquiring their own casino, a la San Andreas.
For GTAO, maybe they could use the new location to give us some kind of story and tell us why we should give a fuck about any of the missions we're offered, because I have plenty of money, meaning my character has no reason to do anything for anyone.
None of this will happen, of course. It would be too smart and enjoyable. Heists will end up paying $100k, tops, with the smaller banks throughout the map possible to rob for a $50k take. There might be some stylish new masks for us (including the gas mask I should've had from day 1), and maybe, if we're lucky, some better housing. Dare I say... mansions?
Even that's too generous, I'm sure. It won't be worth the wait.