Canterlot Castle looked so different from the fortress that Luna remembered. A memorial plaque commemorating the lives lost during the sisters’ battle for Equestria had been placed near the entrance to the castle’s gardens, which were in full bloom with roses, hydrangeas, peonies, rhododendrons, and countless other flowers and plants. Luna was shocked that her favorite tree, the beautiful old oak that she and Celestia had spent so many days climbing as fillies, still stood in the center of the gardens, though it, like almost everything else around her, had changed immensely over the last millennium. The princess sighed heavily and turned her gaze to the full moon as she inhaled the fragrant aroma of fresh flowers, then turned to walk back into the castle.
“Lulu?” The blue alicorn’s ears flicked behind her at the sound of her sister’s voice. Celestia walked up beside her, lowering her head. “What’s on your mind, Luna? You’ve been so quiet lately.” Luna released another sigh and turned to the older princess.
“My… my daughter,” Luna said, her voice faltering as she looked back up at her moon. “I’ve abandoned her. I’m a terrible mother, Tia. I need her. Please, Sister, I beseech you to allow her to come here. I was pregnant when… when everything happened, and I raised her myself during my banishment. Celestia, I need my precious Circe every bit as much as she needs me; bring her home for me, please.” Celestia’s magenta eyes flashed with sympathy, and she placed a comforting hoof on Luna’s shoulder.
“I will see to it that she is brought to you at once,” Celestia declared. “My niece is a princess, and she deserves to be treated as such. I was a fool to assume that you had… lost her… after you were banished.” The alabaster alicorn’s face grew heavy with guilt. “You and Circe will be reunited before the day is out; you have my word.” Luna threw her hooves around Celestia, tears filling her bright turquoise eyes.
“Thank you, Sister,” she whispered, kissing Celestia’s cheek. “My greatest treasure will never be lonely again. I cannot wait to hold her.”
Nightmare Moon got treated so poorly in the show. Zapping her with rainbow lasers into changing her mind is a lazy way to end a villain, most of all because it doesn't resolve her conflict with Celestia. The reason why Luna rebelled was that she felt unappreciated and uncared for and everything stayed exactly the same after she came back from the moon. The only thing that has changed is that Luna's guilt overrode her bitterness and ambition. She could be so much more than Celestia's failure of a sister.
I am waiting with bated breath just who you are going to make her into.