There is really no good answer, it should have been outright locked when it's problematic nature was first apparent instead of just the restricted character creation times.

If they increase server sizes. People would transfer to balmung getting it locked again almost immediately while making several other servers a little smaller
if they just unlock it people would go to it en mass and make it unstable and the way servers are linked all of Aether would become unstable along with it
if they actually did server merges, it would be a nightmare due to housing districts and balmung being over full already would not be a candidate for a merge(keep in mind even the "dead" servers sold out all of their housing in minutes just like the congested ones)

Just don't count on it ever being unlocked because it probably never will be.