If we take a look at the original GTA launched in 1997, it had three cities. At first you played in Liberty City. After earning a sufficient amount of money San Andreas was unlocked (back then it used to be a city not a state). And after completing it you moved to Vice City. In that specific order.
The game featured a top down view. It is often referred to as 2D era (although the buildings looked volumetric).
Liberty City was based on New York City. Obvious reference to Statue of Liberty.
San Andreas was clearly based on San Francisco, California.
And Vice City is definitely Miami, Florida. The name comes from the TV series Miami Vice which was popular in the 1980s.
This pattern was broken when GTA: London 1969, GTA: London 1961
and GTA 2 came out. GTA 2 was located in a fictional Anywhere City.
But the pattern was renewed in 3D era with the next three game titles: GTA 3 (located in Liberty City),
GTA: Vice City
and GTA: San Andreas.
Unlike GTA 1, the latter featured San Andreas as a whole state with three cities: Los Santos (based on Los Angeles), San Fierro (based on San Francisco) and Las Venturas (based on Las Vegas, Nevada). This time Vice City came before San Andreas, not after. It makes sense because San Andreas was way bigger and longer to create. Since 3D cities are much more difficult to design than 2D, Rockstar didn't put them all together but placed them in different games. This gave the time up to several years to work on details of each city separately. And the results were worth it.
HD era started with GTA 4 revisiting Liberty City but in much more detailed manner. HD textures and graphics were magnificent in 2008. The lights, the reflections, the shadows. It was a music for the eyes.
It took 5 more years to create a new HD version of Los Santos in GTA 5. The game was released in 2013. We know that Los Santos is located in the state of San Andreas (take a look at the license plates in GTA 5).
It's been 7 years since then. Rockstar is definitely working on GTA 6. If we look at the pattern, the order is reversed again, like in GTA 1. What comes after Liberty City and San Andreas? Correct, Vice City. Can you imagine the scale of the new city if it takes them so much long to create it?
Of course there may be several cities in GTA 6, hence the delay. But the rumors confirm Vice City. They also say about some Carcer City which we don't know if it will fit the Vice City atmosphere. But maybe this experiment from Rockstar will also be successful.
2D era (original order):
- Liberty City (GTA 1)
- San Andreas city (GTA 1)
- Vice City (GTA 1)
3D era (reverse order):
- Liberty City (GTA 3)
- Vice City (GTA: Vice City)
- San Andreas state (GTA: San Andreas)
HD era (original order again):
- Liberty City (GTA 4)
- San Andreas state (GTA 5)
- Vice City (GTA 6)
Still not convinced? Then how about this. Usually GTA eras are not connected to each other. There are no inter-era references. San Andreas city from GTA 1 is never mentioned in 3D and HD eras, as it later became a whole state. Vice City is mentioned in GTA 4, GTA 5 (billboards, in-game TV) and Episodes from Liberty City (Vice City FM radio station). If it's not a reference to GTA: Vice City from 3D era then it's about a new Vice City in HD era. Rockstar has been hinting us since then.
And of course the first two letters in Vice City would perfectly fit for a GTA VI logo (Roman numerals have been used since GTA III). "VI for Vice City" — sounds like a heck of a great movie.
Compare with the GTA: Vice City logo.