You're completely right, but I still don't think it's a new dungeon, I actually don't think it leads to a dungeon at all.
Going off of the order of the Opening Trailer (which I know is not a legit way of judging and analysing anything, but bear with me) I think the "door" is something you encounter in the very beginning of the Forest Temple, even before the general Forest Area (the demo-area). Again, the trailer seems to imply that the giant "hole" spiraling downward is the first place you see on the surface, and the Sheikah door could very well be at the bottom.
But I'm just guessing here.