Another Concept: The runaway princess.

While I struggle to write through my writers block to write/rewrite at least one of my stories to a point where they are postable, let me just throw out another concept. This one is almost certainly not going to be one I write, but if someone else sees it somewhere, I'd love to get a heads up on where they saw it.

The Concept:
One thousand years ago Princess Celestia held an evacuation of the Everfree Castle, the capital of Equestria, leaving her and Luna alone in the old castle. Shortly there after, there was an eclipse of the sun lasting for a few minutes. Nopony knows what happened there, except that both Princess Celestia and Luna returned from the old castle. However, things were never quite the same. Whatever enemy they had been fighting had wounded Princess Celestia, and had left Princess Luna's hide covered in some kind of sealing symbols.
Furthermore, Princess Celestia and Luna had changed. Celestia had become overly protective of her sister, always keeping at least one eye on the princess. And Luna, had becomed jumpy, having actually hired ponies to sit in her court and watch her for "unusual behavior".
Few ponies took too much consideration for the princess' odd new quirks, concerning themselves more with the enigmatic prophesy by the eclectic Starswirl the Bearded who prophesied that six Alicorns would one day wield a great power against a nightmare; as well as Celestia's enactment of the Alicorn enlistment law, causing all Alicorns to be enlisted into Royal service and training immediately upon their acension.
After a thousand years of the diarchy, however, most ponies had even begun to forget the prophesy. But with the appearance of Princess Cadance, followed by the ascension of five other ponies, the prophesy returned to the public eye.
While enlisting Princess Twilight Sparkle, Pinkie Pie, Rarity, Applejack, and Rainbow Dash had been fairly straight forward, either by promise of prestige, or by promise of duty, or by force of law, things begin to unwind when the Princess Fluttershy ascended, and then immediately went into hiding, rather than accepting her duties.
In a haste Celestia sends the five newest princesses into Ponyville in disguise to attempt to befriend and persuade the wayward princess to dedicate herself to her duty, before Princess Luna loses her tenuous grasp on her mind.