Hey y'all, hoping this thread isn't too dead for anyone to ever see this, but I just wanted to say that the season 7 finale totally changes things.
~~so sorta maybe spoilers ahead, if you haven't seen all the seasons and movies up to s. 7?? you really should go do that~~
So by this point the show will probably never explicitly say so, but for those of us who love the lore enough to theorize about it without the hope of being validated by canon, the season 7 finale presents a great way to settle this whole "7th element" thing once and for all. In the finale, it is revealed that the tree of harmony was created by powerful pony magic, the combined efforts of the Pillars of Old Equestria focused through Star Swirl's final spell. The elements of harmony possessed by the Mane Six are reflections of the elements represented by the Pillars. But whether the elements, in various forms, existed before the Pillars and somehow affected the shape of Equestria's history, or whether the Pillars' actions codified the elements deep into the fabric of Equestria's magic, one thing is clear to the viewers: the defining moment represented by the Shadow Pony's banishment is fundamentally flawed. As the characters come to realize, it wasn't about harmony and friendship at all; it was actually the opposite, ponies turning on their friend.
Therefore, I suggest an alternative view of the events of the Shadow Pony's banishment. The view held by the Pillars at the start of the finale is that they sealed away a terrible evil, sacrificing themselves in the process, and that as their legacy they gave Equestria the seed of harmony, that would grow to reflect the best qualities of each of them and protect Equestria. My view is thus: the Pillars committed a great evil, in how they treated Stygian, and as karmic punishment they were sent away, relinquishing their status as holders of the Elements. This view is implicitly supported by the series in several ways, including a very cool and slightly subtle one: when recounting Stygian's story in Shadow Play, pt. 2, each Pillar narrates the point where they betrayed their own element during the events that led to the banishment at Ponehenge.
And now we get to my point: the event that created the seed of harmony was extremely disharmonious. Each Pillar betrayed their own Element, and all of them betrayed friendship and harmony. That is why, in my view, the Tree of Harmony is flawed, and its seventh Element is broken. The Pillars of Old Equestria were ideally supposed to include Stygian as an equal, and he represents the 7th element. This theory is strengthened when comparing certain events that happened to the wielders of the Elements of Friendship both in Equestria and on the "Equestria Girls" earth. Both groups find themselves sharing their magic with a seventh person, one that they met as enemy, but learned to befriend as they helped them reform. This person (Starlight Glimmer and Sunset Shimmer, respectively) is the one that we identify as the wielder of the 7th element. The similarity of their unconventional circumstances is, perhaps, a cosmic attempt to correct the wrong of Stygian's exclusion by allowing the holders to prove their ability to accept even a former enemy as a friend. In short, even without a proper, physical 7th Element, the Elements of Harmony eventually attract a fitting individual that channels the "broken" 7th Element.
Lastly, you might justly argue that I still have to opine what that 7th element is. I think the answer to that is pretty simple, but allow me first to indulge myself be getting something out of the way. When discussing a 7th element, especially in relation to Starlight Glimmer and Sunset Shimmer, the word "forgiveness" and the like are often thrown around. "Forgiveness" is not their Element, simply because the Mane 6 all forgive them. Rarity's Element isn't "getting stuff", because she is the one giving stuff away. Fluttershy's Element isn't "being pushed around" or "getting lessons about self confidence", because those are all just ways people around her react to her or things that happen to her. Fluttershy's Element is "kindness", because being kind is the thing Fluttershy does by default. It is her main problem-solving mechanism and the hinge of the way she sees the world. So, while Starlight Glimmer and Sunset Shimmer's stories, that are about them doing something bad and then reforming and being forgiven, do contain clues to the nature of their Element, they don't mean that their element is about people forgiving them.
The best way, in my opinion, to determine a pony's element is to see how she approaches problems. When you look at the way that Starlight Glimmer approaches situations throughout seasons 6 and 7, I think you'll agree that it is very fitting to declare the nature of the 7th element to be exactly what it is alluded to be in this twit by MLP animation director Ishi Rudell: Empathy.