I would start by attempting to alleviate the biggest issue first which is that everyone in NA uses Aether as the PF hub for high-end content. If it is at all possible to create a cross-DC PF and DF, then this would help immensely.

Another thing that could help a lot is restricting access to Hunts on other Data Centers. A trains and S ranks create a ton of traffic due to the rewards they provide, and either nerfing their rewards or preventing reward drops when you are a "Traveler" could help alleviate some congestion.

After that, creating some kind of FC transfer feature would be good for some people to move off of Aether. It may not entice everyone but, for those whose primary interest in the game is Roleplay to be able to move to a low-pop server without losing their FC and its progress would be a great feature. Dynamis becoming the de-facto RP Data Center makes sense if they can't make my first two suggestions work to balance all four of them.