Loosely based off It's A Wonderful Life.
After Twilight hurts two of her friends within a short time of each other, she decides she is a threat to them all. She runs away from them for their safety, vowing to never use magic again. Along the way, she encounters a familiar face who claims to be able to show her just what her life would've been like without her magic.
She agrees, and is sucked into a twisted replica of her home. A place where the moon shines forever, the forecast shows a 20% chance of chocolate rain, and any one of your neighbours could be a shape-shifter, ready to suck the love right out of you at a moment's notice.