You and I both. I've only been to Sarasota, Orlando, Newark and Tampa.
I have posted in here before but a new Vice City would be great. I personally am wanting this new city to be around the same size as the GTA IV version of Liberty City as the size of that city was perfect. I am hoping that if they go with Vice City that there will be much more islands as with Miami, I seem to love islands in games.
In addition, I would like to see a new rendition of Las Vegas (or Las Venturas in the GTA Series). It would really be cool to see. Las Venturas was a relatively small city in GTA San Andreas but a bigger version of it in the next GTA game would be great. I'm thinking that Las Venturas has greater possibility than say Los Santos as Las Venturas is mentioned a lot more in GTA IV although this conclusion means nothing, Rockstar have always done this, mentioned the next city along with various other locations to trick us and to get us to start thinking about it.
I have always fancied a European location but it seems less likely for a GTA game to be set outside North America or the USA as there has only really been one GTA game that has.
If it were to be an American or Canadian city that was chosen, I would love to see a depiction of:
Las Vegas, Nevada
Chicago, Illinois
Miami, Florida
Toronto, Canada
Atlanta, Georgia
Elsewhere I would love to see a depiction of:
Paris, France
London, United Kingdom
Glasgow, UK
Berlin, Germany
Naples, Italy
Tokyo, Japan
Mexico City, Mexico