Here is my idea of a possible episode or two---
I think that an interesting episode theme would be that of the telling of a social/love triangle relationship that could exist between Princess Celestia, Princess Luna, and Twilight Sparkle, and how it may have formed. As part of the story, there would be a lengthy flashback to the time when Princess Celestia and Princess Luna were in a mutual and loving relationship with one another. Then it would be shown just how jealousy, as well as other undesirable qualities, began to overtake Princess Luna. These negative traits proved to be too much for Princess Luna, and she allowed them to cause her to deny the rising of the morning sun, leading to her banishment. After the flashback, the story would wind up showing how Twilight Sparkle came to understand the significance of what happened to Princess Luna/Nightmare Moon because of Princess Luna's emotional upheaval. This would be the background material for the confrontation between Nightmare Moon and Twilight Sparkle shown in the second episode of the first season. Although originally protagonists, over time Twilight Sparkle and Princess Luna would begin to develop a more friendly, almost romantic relationship between themselves, thus completing one side of the love triangle. The moral lesson of the episode would be that little girls, as well as adults, should be aware of just how strong emotions can effect one's judgement, and spill over into one's relationships with others.
And Yes ! I agree that there should be more episodes about Princess Luna and
Nightmare Moon, maybe one episode showing her side and feelings about the love triangle !