For those that aren't familiar with my "Radio Theory", you could read it below. The "Radio Theory" is something I came up with way back when, around the time Vice City was still in it's prime. The basic idea of it is that if you listen to the radio stations Rockstar puts in their Grand Theft Auto games, that you'll find out what the next location is.
After only playing GTAIV for roughly two hours, I've heard them mention Ohio and Las Venturas. So if my theory is correct, then the next location will be either based on Ohio, in Las Venturas or possibly another place that is mentioned on one of the radio stations GTAIV has. So keep your ears open and listen for those locations!