Okay, this is a very rough estimate, but hear it goes. I think the main character of GTA 5 could be Jerry Kapowitz, and it will take place in Vice City.
Anyone who has beaten the story of GTA IV and its expansion packs knows that Ray Bulgarin's diamonds were the meeting point of all three stories. Also, the diamonds were related to the final missions in all three. Jerry Kapowitz is a homeless veteran in Liberty City. He makes a small cameo once while hanging out in Hove Beach, he begged Vlad Glebov for some spare change, while walking with Niko to his car. (http://gta.wikia.com/Jerry_Kapowitz).
In the final mission of GTA IV: TBOGT, Luis bumps in to Jerry Kapowitz. When he gets up he notices a small bag and sees the diamonds in it. Later on the radio, you hear about the diamonds, their value of 2 million, and Jerry's plans to move to Vice City.
Although this does not follow GTA's normal scheme, heres what I think:
Jerry is a once low-life hobo living in the slums of Liberty City. He finds a bag of diamonds and uses his newfound fortune to move to Vice City and live the good life. He moves there and all goes well until Ray Bulgarin's men (or someone else from Liberty) decide they want their diamonds back. They get their stuff back with Jerry only getting a fraction of the money he was supposed to. He is now a nothing once more and has little cash to do much with. He gets involved with the wrong people and has to fight his way to the top of Vice City to gain back his fortune.
Even if that isn't the story of GTA 5, wouldn't that be cool? If this is in any way involved with the story of GTA 5, I'll be amazed. :-)