So, for all who've watched the Season 4 Premiere of MLP: FIM, I bet you're all wondering about the "Box" that was featured in the Premiere (37:55). As Princess Luna said, "Six locks, six keys." I'm pretty sure we got the main idea about the box being about the Mane Six. But physical or immaterial, six "key"s are needed.

Twilight staring at the chest cropped S04E02

Twilight in awe with the "Box".

My hunch is that the box will finally be opened in the Season 4 finale to reveal... Nothing. But for the mean time, the finale's preceding episodes involve the Mane Six  developing their friendship and all those "Dear Princess Celestia," epilogues. And then, nearing the season finale, Twilight's alicorn duties become more and more heavy and her and the rest of the Mane Six's friendship gap become more and more noticeable. Princess Celestia, being the boss around Equestria, notices it too (with her all-seeing eye and all), and gives Twilight and the rest of the Mane Six a task that they have to accomplish together (that could involve them having to carry around the "Box" or something similar to look for its keys). This undertaking gives them time to bond and realize how far they've drifted apart and they mend their friendship. They share another one of their perky group hugs and then these six random keys materialize out of thin air and get inserted into the "Box"s keyholes. And they open with a bright and blinding light to reveal nothing. But they don't care and realize that the main goal was to reflect on their friendships and to strengthen their bonds (and this whole thing could've been all conducted by Celestia, the "evil" mastermind).

*EDIT- (Spoiler for those who haven't watched S4E3 Castle Mane-ia) After watching S4 E3, apparently, the "Dear Princess Celestia," epilogues aren't happening anymore, and, instead, they (Mane Six) will be writing on their own diary similar to Princess Celestia and Princess Luna's ancient diary found in their old castle. But I'm guessing every following episode after S4E3 will end with a "Dear Diary," short.

Link for Castle Mane-ia:

But, yeah. That's my hunch. And then all bronies will go crazy again during the wait for Season 5.

If there is one.

Hasbro... I'm watching you.