Even though a return to Vice City has been talked about for some time, this information can not at all be authenticated at this time.
The game world will be a lot bigger that any previous GTA game. "The game world will be much more expansive than just those nighttime city scenes with the neon over and over again". Full day-night cycle, weather, lots of different neighborhoods and much much more. They are really trying to create an incredible sandbox!
Reports are also saying that the shooting & fighting in general are said to be very realistic and more like an action movie. You will be able to use the environment in a totally different way, for example, kill someone with a fan or drown someone in a fish tank. They've integrated different types of minigames like gambling, karaoke and dancing. Also a lot of cop mini games like surveillance and safe-cracking.
For the first time in the GTA-series, multiplayer will be available and you will also be able to choose a side, create your character and form your crew!