I think the new one [by the way it is way too early to be talking about this] is going to be in London.
- Change of location [European country]
- I don't think the feel of the Miami city would be good enough for a GTA on the next generation - too clean and a bright city
- London goes with the dark, grity feeling they captured in Liberty City
- It's a decent size city that R* would be able to capture and emulate in the style of GTA
There were some other reasons I thought of but just woken up and can't recall them.