Okay, another random thought:
What if Princess Celestia and Luna didn't just give all their responsibilities to Twilight but were like "Okay, we need to elevate two Princesses: one for the sun duties and one for the moon duties. This job is too hard to do alone."
Twilight Sparkle would seem to be the obvious choice for Moon Princess because she is night themed and loves the stars. This would leave the question of who would become the day princess. Will it be one of Twilight's near and dear friends? Or Sunset Shimmer? Or a total stranger?
(I say Twilight "seems to be the obvious choice" because I would love a twist where the new Moon Princess turned out to be also-star-themed Starlight Glimmer instead. I feel like her personality would make her better at the job than Twilight. Maybe her friend Sunburst gets turned into an alicorn and becomes the Sun Prince. Twilight could become the royal librarian and lore master, she would love that.)













