That would actually be really neat to see, makes me wish they kept the three city states seperate.

"Help finance city improvements by selling your crap on our market boards!"

maybe once every 6 months have a few minor city upgrades based on the amount of gil earned through taxes. Then you could see visible differences between servers and who is selling what where. It could be minor things, upgraded atherite shards, different lighting fixtures, different skin/texture for the roads, etc. It would be a lot of work on the devs end to set up though.