They seem to be avoiding the biggest cities in most of the games. I mean, there's no Midgar, Esther, Zanarkand, or Archadia yet.
I'd put money on Mhaura the next XI city.
One thing is for sure, anything from FFVII, X or XI is going to be congested as fuck at launch. I'd love to go with Selbina or anything from FFVII but it's setting yourself up for pain at launch.
When they close servers off due to being over-crowded, is there no world pass type system to get your friends on it? That's a bit shitty if not.