From past experience (v1 Early Access in 2010), SE will make worlds available and unavailable depending on congestion in certain zones. A world that is "full" now might become available again later.

When I started, Besaid was "full", so I chose another. (iStory. er, Istory.) Many days later I heard it was available again. (You only saw server availability after creating a new character.) (Yes, that's the world's smallest violin you hear playing in the background...)