Yeah, I think they mentioned something about having multiple ‘directions’ (e.g. Overworld above Krystara) they could go in! 
Can’t remember where or if it was a player/speculation.
I do remember Sirrian mentioning the need for something after all 34 classes had been allocated on stream, and saying probably a new game mode.
http://www.twitch.tv/infinityplustwo/v/441113203?sr=a&t=3262s
Time stamp 54:22 (I tried to make it work in the video, but think it only works for the link).