I think it's pretty obvious by the end of Season 2 we find Carole Arna (Helios Academy teacher). She then sends us to find Professor Arche who was the founder of Helios Academy, a Void Hunter that's still alive exploring the depths of Hollow Zero. That's where Wise and Belle's training from all of Season 2 comes in to make them be able to go further into Hollow Zero. Story can then branch off on mysteries of the Hollow, Ethereals and the old void hunters.