If FFXIV closed off the main zones via rank 30 agro mobs, so you have to be rank 25 to get to other cities, then new players would essentially stay in their "home zone".
Then you change the levelling curve so instead of hitting 25 in 2 days it takes a few weeks, but getting from rank 40-50 takes a lot less. This way people spend a fair bit of their starting time in their home city and get used to it.
Then have "universal wards" instead of city specific ones.

And now your population is balanced between the three cities.