My headcanon is that Sunset will to return to Equestria and becomes Celestia’s prized pupil again. Celestia was happy and also proud of Sunset, seeing how she molded into a great pony being that Celestia intended for her to be. During her time as her student, she shared her knowledge with Celestia about the human world’s magic and how it works. She studied more of Equestria’s magic thanks to Twilight’s help. Pretty soon Celestia decided its time for her to become a alicorn which she really truly deserved now that she was ready. Celestia crowned her as “The Princess Of Magic”

i was reading the journal of the two sisters a couple months ago and i notice how magic was truly important back then and a question popped into my head “Why isnt there a princess of magic?” Celestia is in charge of raising/lowering the sun and day, Luna is charge of raising/lowering the moon and night, Cadance is charge of spreading love and ruling of the crystal empire, and Twilight is in charge of fixing friendship problems. But whose in charge of magic? If Sunset Shimmer were princess of magic, she’ll fix or solve any magic problem that is occuring because it’s in her field. Twilight would work too but she has her own problems to deal with. I do think Equestria does need a princess of magic.