I don't know but the next one is rumoured to be in the same vein as the "GTA London" add on pack for GTA2. However it will be called GTA 4 and on a totally different GFX engine (look better)
Also expect the worlds to take a similar jump in size to that of GTA3 - Vice City - San Andreas, i.e. Fooking HUGE! will probably be a sizable section of Northern Europe!