Mane 6 plus 2

Again, this is just my headcanon and I won't change it.

Yes, I see Sunset and Starlight a perfect candidates for the elements of harmony.
Sunset already does seem like one, considering the final fighting scenes in recent EqG movies and she really deserves it after that all. She proved she understands forgiveness and she forgives every little bad thing said to her name (for a former villain, she's too forgiving). And don't forget that forgiveness is one of the important traits in friendship too! While Mane 6 as a whole are forgiving, there's missing somepony, who would represent this trait / element separately. Sunset's development convinced me to consider her being worth to represent this missing element.

As for Starlight, I don't care how many of you hate her and the idea of her representing an element is irritating or hilarious for you, I have my opinion to her as a character. And yes, justice is what she represents, or at least tries to represent a whole time, since when she was "evil". She's an personification of the justice itself. All she ever cared about was to give everypony the same chance to make a true friends and equality. She wanted justice in everything. And while it doesn't look like she'll become a Mane 6 member, she became VERY close with them (after everything she came through in the season 5 finale) and it doesn't make her any less worth to be an element. Plus, she's a Twilight's student now, just like Sunset. I know that justice is also a part of honesty -Applejack's element, but it's not really the main filling of this element. Justice is as important thing as the other elements. And while Starlight can never be an element in canon, she simply is in my headcanon and always will. Therefore I hope you'll respect my opinion and I'll respect yours.

Just one thing -this is my last addition to the elements of harmony and my final decision (unless the canon wouldn't add another). I just feel like these two elements are missing despite they're important. And these two ponies represent them perfectly.