I made a similar post on other forums, so I might as well here too. These are my reasons for thinking it's San Andreas.
Next Stop: This one is pretty unusual in my reasoning. San Andreas is the only game in the entire series to have trains you could drive. It was also the only one where trains became useful, and were a side mission. Seeing as trains have fixed routes, you could easily judge the 'next stop'.
Opens Everywhere March: In San Andreas, there is an advertisement for a religion based theme park, called 'GloryHole'. The GloryHole theme park is meant to 'Open everywhere in March', or something to that effect IIRC.
Mountains: Only game in the series to feature mountains is San Andreas. There was Mount Chiliad, in the south of the countryside.
Seagulls: This one could be either Vice City or San Andreas, however, I think it's more likely to be SA. Why? There is attention drawn to the seagulls during the story. It's the mission where you have to find Mike Toreno. The first thing said is 'Mike can hear seagulls'.
Liberty City, It's Over: Catalina says 'It's Over' to you before leaving for Liberty City with Claude. This is at the end of the mission where you're racing in the countryside. I think the mission is called Wu Zi Mu or something like that.
Like I said, this is just my theory on things. It could be way off. It could be an entirely new city. We don't know yet.
