Theory: The element of Starlight

Lets go over the basics again - We started off with elements of harmony which originate from the tree of harmony, which holding all the elements again, revealed a chest, which turned out to be a castle seed & it grew into a giant castle with a big interactive table, cutie map, in it, which sent the mane 6 on an adventure to the equal town, where they made a powerful enemy Starlight, who came back, altered the past & led Equestria to its doom, so Twilight convinced Starlight not to do that & Starlight became her apprentice or protege of some sort, so now she's studying under Twilight is part of the mane 7...and now you're all caught up, in case you've never watched the series.

Now, time to connect the two dots, the tree of harmony & elements of harmony from earlier, and the pony the cutie map led the mane 6 to, Starlight. Starlight's far known for her OP magical abilities & for her friendship studies under Twilight. While, setting on the same journey as Twilight once did, there's still a lot for Starlight to discover. Such as her element of harmony, which I hope to see come this season finale. Obviously, she's gonna face a challenge of some sort come the end of the season & that'd be the perfect time to reveal her element of harmony & whatever her cutie mark can mean besides the obvious "magic". 

Could the tree of harmony perhaps be fueling its entire energy into Starlight? Who knows...

But for now, lets see what we can gather up for this theory. As I've said before, these are merely just theories, so there isn't a 100% chance of them being true, but these are possible interpretations of what could be.

Exhibit A

As you can see, while surrounded by 6 elements (which are all already to our knowing), there seems to be a big ball, right in the middle, possibly hinting at another cutie mark. This, however, is rather early in the series, so what it was back then wasn't probably it, as there wasn't even a mere thought of Starlight in the writers minds, so of course this can be later interpreted the way if the writers wanna go that direction, but it's merely a part of the pony universe of interpretation right now.

Exhibit B

The tree of harmony. While, seeming complete at first, could there be things about the tree of harmony we yet don't know about. While, it seems the tree of harmony is quite full of trickery, it did send the mane 6 on the way to the equal town & provided the cutie map that acquainted Twilight & Starlight in their fun adventure through time.

Exhibit C

Now, onto the cutie map. The thing which has had perhaps the most direct link to Starlight & something that could possibly bring her destiny & element of harmony to be. The cutie map, working based on recruiting ponies to a certain location by their respective cutie marks, the high end of the Equestrian magic. We'll probably see it recruiting Starlight one day & with that, we'll be one step closer to discovering whatever besides magic & being OP could her cutie mark mean.

As for now, the story does seem to be leading up to Starlight, so if she does represent an element of harmony (& considering her role, she probably does), we should have the answers very soon.