I just want to say that many fans believe it would be san andreas but i think that they are wrong.
In the end of TBOGT we see Patrick goes to airport. This can mean that he is next gta protoganist. He goes to vice city. Why to Vice city? Mainly because There are no evidence that San Andreas would appear in the next game. But Vice city otherwise first we have a message from Roman that they( with nIko) might go to Vice city if they have money. Niko in the end has more than a million dollars and the ticket would only cost maximum 1000$. Next we have Jerry a tramp who got the diamonds. He goes to vice city and you can make a big story out of it.
Patrick would suit the main role quite well. He would be different from alll other protoganists and it would make him have humour. He might call to Liberty city to know how things are going with all this mob running.
Also San Andreas would perfectly suit the grand Finale of GTA IV era. Ofcourse after SA they can make more games. But like the old San Andreas it is a time for experiments. Making skills and all that sort of stuff.
So that mostly all reasons i have for vice city being the next location.