Same universe? Yes.
The GTA Universe is basically all parallels of the big US cities.
Liberty City (New York), Los Santos (Los Angeles), San Fierro (San Francisco) and Las Venturas (Las Vegas). And Vice City... I think that's Miami.
They haven't done Chicago yet... or Dallas... plenty of other US cities to pick on.
But yes, it will be the same universe again, GTA doesn't need to create a new Universe.
Honestly, I would love it if they went back to the 80s again, like Vice City. GTA Vice City is a legendary game, but I would really love to see a computer game that did the 80s on the kind of massive scale that is possible now.