My theory on sunset shimmer, starlight glimmer, and Flurry heart.

[Storm Fizz 0]

In the season 9 finale, we see Twilight Sparkle all grown, and she seems to be the only princess in the castle. I think Twilight decides to promote two more rulers for balance, but also to take on some of the old princesse’s duties because it may be tiresome for her to do on her own. I think she chooses Starlight Glimmer and Sunset Shimmer. I think this is because although they may have done wrong in the past, they set it right by doing the most heroic and wonderful actions. Starlight saved Equestria from Queen Chrysalis yet again, and Sunset  saved the Human world through the mirror  and Equestria from Midnight Sparkle and they may have eventually done more things like this and led her to finally give both of them a place in the palace as princesses. My theory is that  Starlight takes on the duty of walking in dreams like Princess Luna used to, and Sunset Shimmer learned to raise and set the sun with her new-found powers as an Alicorn, and Twilight rose and set the moon with her power. (Starlight and Sunset are done by me)

[Reiki Knight 13 1,461]

I can see that happening. Sunset could have also become the first human alicorn as well it could have gone ether way with. It's a true shame the mystery of human Sunset was never solved...