i doubt rockstar will pull a valve and stop making gta games. the popularity of gta will definitely delay gta6 and also the dev time required for it.
the real question is will the game have 1 city or 3. i hope they go 1 but its fully packed and i hope its not NY.
It's pretty much confirmed in my opinion in the way of design that it'll be either Vice City or a whole new fiction city. When you think about it by a design standpoint, we already have HD worlds of Liberty City and Los Santos. So to come back to Liberty City and come across already visited landmarks/points-of-interests, it'll be less interesting and have less explorable replay ability. To achieve more impact and surprise I can guarantee it'll be a brand new HD Vice City with some nostalgia attached to it!