People should get incentives or something for selling things in that city related to the guilds that lay within..
What though could be offered, I haven't the slightest idea, but it would hopefully help with the spreading out of things.

It would be interesting in my opinion to be able to go to Limsa knowing that I would find the best prices on heavy armor and weapons, go to Gridania for my leather armor, and go to Ul'dah for my cloth and jewelry related stuff and so on.

I think the biggest obstacles for spreading out though is that we don't have a very good mode of transport.. We are limited to the amount that we can travel not so much in time we have, but in how much anima we have left.
We can't go rent a chocobo and go to a neighboring city in 10-15 minutes, instead we have to spend anima that takes hours to get back... This strongly discourages massive amounts of travel in my opinion, and so keeps us generally in the same areas ;v

I'm sure this has all been said before a billion times, but it feels relevant