Ah good, a discussion I can actually join in (I don't have an account with any of the forums).





I actually came up with the theory that if Sunset finally achieves her past goal of becoming a princess, then she'd be the Princess of Protection with the Elements Courage and Empathy. I originally went with Protection as her Element, then found about about Empathy being considered. I also found a suggestion from a fan that her Element could be Courage. Then I realized that both Courage and Empathy worked with Protection, because protecting others from danger takes a lot of courage but you also need empathy to understand how to best protect someone from certain dangers and pains without hurting them.







Sunset's fury at Human Twilight after the motorcross race incident in Friendship Games was what inspired the Protection idea, since it's obvious that Sunset is now VERY protective of her friends and doesn't like to see others hurt. Plus there's the fact that she was unwilling to harm Spike and in fact wasn't shown physically harming anyone in the first movie before she was corrupted. It could be because she never liked anyone in general getting hurt, and her desire to be a princess stemmed from her desire to protect others: as a princess she'd have plenty of power to protect pretty much everypony (maybe even everyone, if she was considering protecting other species as well as ponies). However when she became obsessed with becoming a princess she started to forget why she wanted to become one in the first place.





Then after being reformed she started to gradually learn the Elements she needed to become a princess. First she had to learn courage, since we saw her being afraid of being accepted by even the Humane Five and Pony Twilight during Rainbow Rocks. Plus she picked up on the problems of the Humane Five constantly arguing about certain details early on, but lacked the courage to speak up until around the end of the movie. Her ending the Humane Five's fight and joining in the music battle was her finally gaining the courage to do the right thing no matter what. Like Albus Dumbledore said, in reference to Sunset ending the Humane Five's fight: It takes a great deal of courage to stand up to your enemies, but a great deal more to stand up to your friends.





She then needed to learn Empathy, which is were Friendship Games comes in. After all Sunset was right in telling off Human Twilight for messing with magic, given that people could've died during the motorcross race incident, she just went about it the wrong way. Basically Sunset had to learn the hard way that she has to be more empathetic with anyone causing danger, especially if they were causing it unintentionally. We can see hints of this when Sunset looked very regretful after Human Twilight ran off in tears, since she realized that while she was right to be protective of her friends her lack of empathy caused her to unintentionally hurt Human Twilight which she never wanted. Then when Human Twilight got corrupted by magic Sunset was able to use empathy to stop her without hurting her (though it was mostly thanks to Human Twilight realizing that Spike was upset over what she was doing).





As for what could trigger Sunset's princess transformation...well, Twilight got her transformation when completing a major Magic spell that required a deep understanding of Friendship. So Sunset will probably have to do a huge Courageous act while showing plenty of Empathy. Maybe risking her life to save an enemy who'll nearly kill himself/herself during his/her attempts to gain power, due to realizing that the enemy doesn't deserve death despite wanting power. Or something similar.





Starlight I already figured would have Forgiveness as one Element. If she has a second Element I haven't figured out what it could be yet. Maybe Fairness? Of course this will depend on whether Starlight could be another princess or not. If she was I'd say she'd be like the royalty equivalent of a judge, so going by your idea maybe she could be the Princess of Justice.







But either way, I think Forgiveness fits Starlight more than any other redeemed enemy because she had to learn that lesson the most (not counting Princess Luna). She wasn't able to forgive Cutie Marks for taking away her only friend, leading to her Anti-Cutie Mark village, and then she couldn't forgive Twilight and her friends for causing her equality plan to fall apart leading to the time travel mess.







But then there was the whole matter with Sunburst. In The Crystalling it was obvious that she didn't want to see him because she was afraid that he'd reject her for her past, rather than still holding a grudge for his having left her after getting his Cutie Mark.





And then there was Trixie. It was pretty obvious that Trixie pretending to befriend Starlight to show up Twilight pretty much broke Starlight's heart. Yet after a short speech from Twilight Starlight quickly forgave Trixie and rekindled her friendship with her.







Because of those two details I think after Twilight forgave Starlight and offered her friendship Starlight learned that forgiving others and trying to start a new life was better than continuing holding a grudge, but still had issues with forgiving herself.







That's when To Where and Back Again comes in. Starlight had to learn to forgive herself enough to give leadership a try again, which she does by the end of the two-parter. And there's the fact that she forgave Chrysalis and offered her friendship despite everything, while everyone else (even Flurry Heart) looked ready to pretty much kill Chrysalis.







I guess for her second Element...again maybe she just has to further learn how to inspire equality without enforcing it. Yes she now knows that forcing everypony to be the same causes more harm than anything else, but she may not know how to encourage equality without her Cutie Mark-removal spell. So her final act will be learning to promote equality without taking away individuality.





I think having more than three princesses works out too. Celestia controls the sun and court issues, Luna controls the moon and protects Equestria as everyone sleeps, and Twilight encourages friendship inside and outside of Equestria further increasing harmony. But there are other roles that could be filled in.







Sunset could be like a 'head of the army', like how kings/princes lead their armies into battle, only she would defend with empathy more than weaponry (weaponry, like spells, being a last resort). She could even come to Equestria to help Twilight out with protecting it. While Twilight promotes friendship to get some troublemakers to realize they're wrong, Sunset uses empathy to understand why other troublemakers are doing what they're doing and offer a better solution.





