I'm thinking that Grand Theft Auto 6 might take place in Vice City. If you look at the orders of the games in both universes by year you can see this:
3D Universe[]
- Grand Theft Auto: Vice City Stories takes place in Vice City in 1984
- Grand Theft Auto: Vice City also takes place in Vice City, but in 1986
- Grand Theft Auto: San Andreas takes place in San Andreas in 1992
- Liberty City Stories, Advance, and Grand Theft Auto III takes place in Liberty City between 1998 and 2001
HD Universe[]
- Grand Theft Auto IV, The Lost and Damned, The Ballad of Gay Tony, and Grand Theft Auto: Chinatown Wars takes place in Liberty City between 2008 and 2009
- Grand Theft Auto V takes place in San Andreas in 2013, even though the prologue is in 2004
In case you missed it, The HD Universe is a backwards version of the 3D universe. So, it is possible that GTA 6 and 7 could take place in Vice City.