Theory on BotW 2:
So, here are a few things BotW has going for it.
Unanimous praise
Relatively huge open world, the likes of which we'd never seen before
Pushing the boundaries with things unique and new
It got a sequel; a sequel that, mind you, is much darker in tone than the original.
Sound familiar? Well it should, because I'm also describing Ocarina of Time. And that sequel is Majora's Mask. Let's recap on that game, shall we? The Skull Kid hijacks your horse and sends you down a large pit, which leads him to Termina-a land that was underneath Hyrule all this time, almost an alternate dimension of sorts, where the characters you saw in the first game are reappearing here.
Now, let's recap on Breath of the Wild's sequel. At the end of the trailer, we see Hyrule Castle raising into the sky. What could be beneath it? Well, I have a theory-a pit, similar to the one Young Link fell down in MM, that leads to the same destination-Termina.
My theory states that, if it is true, we will be traveling between Hyrule and Termina at will in the BotW sequel.