In my opinion, the only kind of episode (besides a more in-depth backstory) that would really do Celestia's character justice is more of a slice of life episode. There is a children's book called Princess Celestia and the Summer of Royal Waves that does have a bit of action but focuses mostly on Celestia's role as a teacher. Outside of her usual executive, ceremonial, and sun-related duties, that is her occupation. Maybe the writers could could use that under-appreciated aspect of her character to write an episode.