Remember how (pre-E3) Aonuma mentioned that he had a certain "twist" on the open world genre? Did we actually see anything which could constitute that "twist" in all of this footage?
I'm betting that there will definitely be a time travel element, and having something as dynamic as time travel arrows which bring whatever they hit into the past/future/whatever would be ridiculously cool.
Also the towns are likely related to time travel. Imagine if there are actually two versions of this overworld- present version and pre-Link's 100 year nap where Hyrule hasn't yet been destroyed. That would definitely be an awesome "twist" that I can imagine them doing. Thinking about it even more, Aonuma mentioned the minimalist music style had a lot to do with the ruined, desolate atmosphere... imagine if upon going back to the past we get a full overworld theme similar to previous Zeldas! How incredibly epic would that be?
They've shown enough to get us excited but they've held a whole lot back. Just from the standpoint of the world's geography, we've seen only a tiny portion. The plateau, with its snowy peak, water bodies, forest, shrines and ruins, is between 0.01 - 0.02 of the totality of BotW's handcrafted Hyrule.
They've obviously held back gameplay mechanics as well. Anything that involves doubling the world size (like having a past version of Hyrule that you either reveal in pockets like SS's timeshift or travel to) would be absolutely crazy. I'd seriously have to wonder if Aonuma and co. are insane if they decide to take that on.