Remember the box at the end of S4's opening?

I've thought about that too. It would be a cool twist indeed, but unfortunately, I think it is very unlikely. Since the Elements of Harmony can no longer be utilized as a tool to save Equestria, the box probably contains something that will replace them, to fill up this new void.

On the other hand, it seems the writers intend to change the entire setting of the MLP universe (The Season wide story arch, a new villain, the Journal of the Mane 6, No lebih Elements, the changed status of the Everfree, where ponies often hang out these days indicates this), so I would't be surprised of a major twist at the end of this Season.

However, Pandora's box is a negative object in mythology and I don't think the pohon of Harmony - the manifestation of a powerful white magic - would produce something that would unleash some evil forces. Interesting assumption nevertheless.

What he berkata ^

Oh! Box box box box what's in the box? Box box box box what's in the box?