There are two main things I think need to be done just to allow people to hang out in their city-state of choice.

The first is a proper search system, which is coming with the server rebuild. If you want a group for a dungeon, for EXP, or for any future content that will require one, you currently need to be where other people are because no one uses the confusing search system and just shouts instead. You're not going to see such a shout or get a response to your own shouts in, say, Gridania. Once people can look for you there though, you'll be more free to do so and still get invited.

The other is somehow fixing the market wards system to make it so centralization is no longer advantageous. Linking them all together and then making it so when you leave you exit back out to the city-state you were in, for instance, would accomplish this. Being able to search all three from the same place and then buy without entering would too, but then that would just be an auction house. Whatever is done, there needs to be a way for people to buy and sell items just the same from separate locations, rather than gathering together being superior.

Both of these things revolve around the idea of making it easier to be wherever you want. Other solutions would involve enforcing distribution, such as how guild marks are currently distributed. Those solutions force people to do something in a specific place rather than allowing them to be where they choose, but control even distribution by giving equal reason to need to be in all three.