Well, let's start evaluating which cities would be possible.
CITY - PROBABILITY (higher is more probable) - SIGNIFICANCE
Liberty City - 0 - Duh!
Carcer City - 1 - Atmosphere and mood may not suit GTA, not vibrant enough.
Vice City - 5 - Most probable. Has been done before
Los Santos - 2 - Not very probable; Los Angeles already done in Midnight Club: LA
San Fierro - 4 - If Vice City is skipped, this is the next most probable.
Las Venturas - 4 - Same reason as San Fierro.
San Andreas (LS+SF+LV) - 3 - It's unlikely they'll do LS, as stated above.
London - 4 - They haven't gone out of the USA for quite a while now, but with the Getaway out of the picture, it may be possible
Old West / Town setting - Not very probable (Red Dead Redemption)
Japan (There has been three Yakuza games now FYI)
We must take into consideration MC
A, Mafia II, and Red Dead Redemption - they are three open-world games also released by Take Two, thus it would be most unlikely for their settings to be used in the next GTA.