OK, I just have to say this:
WHY DOES EVERYBODY THINK THAT GETTING RID OF THE ELEMENTS OF HARMONY WAS SUCH A GOOD IDEA?!?!?
THINK ABOUT IT.
When a new villain appears who can ONLY be defeated by the Elements of Harmony, that will be a giant "OOPS!" moment for the Mane Six.
...Actually, now that I think about it, I foresee a "The Mane Six were the REAL Elements of Harmony all this time" plot point coming up.
That, or the box contains "Super Elements of Harmony".
But it seems the former is much more likely.
Actually, I don't think it was such a good idea. And I'm questioning as to WHY fans are thinking it was such a good idea.
In my honest opinion, the REAL purpose of the Elements of Harmony was to defeat villains.
They didn't use them to overcome ALL obstacles; just Nightmare Moon and Discord, nothing else(at least, as far as I know).
I think that, inside the chest, are the NEW AND IMPROVED Elements of Harmony. Probably called something like the "Super Elements of Harmony" or something.
But even IF it's the Rainbow of Light, it won't replace the Elements of Harmony, no matter the nonstalgia.
...The Rainbow of Light FUSES with the Elements of Harmony, resulting in...
...SUPER ELEMENTS OF HARMONY.
Personally, I think that it's, like the "Super Elements of Harmony" or something.
This is for two reasons.
1: Except for the hardcore bronies, bronies will complain about how the Rainbow of Light sucks in comparison to the Elements of Harmony.
2: Unlike the Elements of Harmony, the Rainbow of Light is a single item. If anything, all the RoL will do, besides defeat villains, is tear the group apart due to them arguing over who gets to use it.