What if…

Instead of sending Nightmare Moon to the moon right after she turns, Celestia decides she cannot fight her younger sister because of how much she loves her. So, after getting roughed up by her angry sibling and dragged around Canterlot in humiliation (by ropes), she is banished to the Everfree Forest. During her banishment, Celestia grows and learns the Elements of Harmony through kind acts she does during her stay, earning each color of the rainbow in her mane for every Element she discovers.