I mean, there's the land around Hyrule, and also the oceans as I was hinting in my comment. Yona is also apparently from another domain, which could perhaps a sea based one? They could expand the current Hyrule via the rivers and lakes as well as an even further restored Hyrule, perhaps expanding some of the towns and villages. As for story, I feel like they keep hinting at the Triforce without actually implementing it fully, so maybe that?

Deeper depths or higher sky also seems possible, though I don't think they'd do that, as much as they feel prototypical to some degree. Seeing the original home of the Zonai would be cool though, if it's still there extremely high up. Perhaps a proper dark world? I know the Depths are a pseudo-implementation of that, but even so.