Going out of Hyrule *would* be cool, but as things stand, there isn't a reason to. As much as I'd love to see them do that, they're still trying to tell a story that takes place *in* Hyrule, so I really dont expect to be able to leave its borders.
I spent over a thousand hours exploring BotW's Hyrule and I still wouldn't mind it being in BotW2. What I personally want to see is an underground (which is kind of what the teaser is implying) that stretches all out under the overworld we already know, with pelnty of new stuff to explore under...