Besides that we all have a feeling on where the next GTA will take place, I have found some kind of hint that Rockstar left in San Andreas. While watching the introduction mission for the billionth time on, I noticed a sign at the airport while Carl was picking up his luggage!!!
LOOK AT THE RED CIRCLED AREA IN THE PICTURE BELOW
Carcer is a reference to the setting of Manhunt known as Carcer City
I researched Johnson as Johnson City and that is in Carter County, Tennessee. This could be a reference to Carl's last name, the countryside in San Andreas, or Manhunt again as Carter is very close to Carcer.
I couldn't find much for Leith Intl, but Leith is part of the City of Edinburgh in the United Kingdom, where Rockstar North is based. (Rockstar is so tricky sometimes :-))
Finally there's Tokyo, which most likely points to the location of GTA 5. I'm sorry if someone has pointed this out before, but I want to make sure people know!
Enjoy my research,