I think it would be really cool and interesting if the cave had some connection to say Conrad, who hasn't gotten a ton of screen time, and could easily be a spark for something that leads to revelations, maybe new twists added to the journey to Skellig. I don't think Miura is going to waste time making this island solely about showing Guts what the new world is all about, while this is certainly going to be in there, other interesting developments will be weaved in.