Centuries after Discord's defeat and the separation of the Royal Sisters, for about a decade now, there is peace and prosperity in Equestria under the rule of Princess Celestia. Her governance is wise, benevolent, and remarkably efficient. However, it's not all so simple. Celestia may be an immortal alicorn bearing the wisdom of centuries, but along with this wisdom comes a heavy burden of memories and decisions. After centuries spent toiling away on her own, something had changed.
Few know it, but the creature they see as their Princess in public is, in fact, a body double. A mysterious, shape-shifting, insect-like empath creature who wears the guise of the alicorn to handle all the things she would rather not. How this creature came to be and how Celestia came to find it, not even the ones priveleged to know the secret of Osmosis the shapeshifter know.
While Osmosis' shenanigans have taken up a good amount of Celestia's worries for the last decade, they were also her only form of proper social interaction. Nightmare Moon's return is on the horizon, and after such long neglect, Celestia's senses are unfit to seek out candidates for a trained protege - a pony capable of reuniting the Elements of Harmony and wielding them when the time comes.
50 years remain to find a fitting family from which to find one. Whatever Osma is, her ability to stay on track makes that an adequate timeframe...
Stray elements of Romance and Comedy, but enough to make it into the tags IMO.