These guys will have the game in their hands, complete it 100%, and still look for the other two cities
I was keeping my hopes high until ROCKSTAR THEMSELVES made an official statement. YES, they didn't reveal other two cities in SA in first trailer, but I don't recall them making an official announcement saying GTA: SA would just be Los Santos.
They are creating LA in a much grander and detailed way. As they say it's their largest yet. Go to Google Maps and see how big LA is, and add countrysides and mountains etc to that.
This is the perfect GTA, concentrate assets on one city means more detail and having the surroundings means it won't feel claustrophobic.