Part of what contributed to Ul'dah becoming the hub is that following the main story sends you there.

Would much rather stay in Gridania, but so many things are causing people to gravitate to Ul'dah. And now that Ul'dah has become the gathering place coupled with the majority of the economy being there, it is going to be hard to split things back up.

Economically, the only thing I can think of would involve adjusting the tax rates. Maybe something along the lines of steeper rates for certain items in some locations, and in others giving additional gil for selling the item there. As an example will use lances. Say you want to sell a lance for 10,000 gil, if you were to sell it in Gridania you make 12,000 gil, while if you sell it in limsa you make 10,000, but sell it in Ul'dah make 8,000. I do see some issues with this, such as the specific item types only show up where they make extra, and a few others I can think of if expand upon this idea.

As to socially, that would be significantly harder without introducing some form of world chat, or better search means.

As to the idea of using R30 agro mobs to block off the paths to other cities, remember the main quest that sends a new person to Ul'dah is currently a R18 mission.