http://www.novacrystallis.com/2015/0...ready-decided/

Naoki Yoshida: “The team already fixed what world it’s going to be and what the special feature or focus will be. The first task for 4.0 is to draw the storyboard for the next opening scene. You maybe still remember the opening scene of 3.0 with the dragon. I have written the concept for this storyboard a year and a half before launch. Right now I have time for the next one and the end of the 3.x scenario is already fixed, too.”

Not much, but it seems it will be another 1.5 years till the next expansion. Swimming will most likely be the next feature.