Celestia and Luna married the mane six, now they have a harem and a honeymoon to deal with. In all honesty, happily ever after isn't all it's cracked up to be. At least there'll be room for orgies.
To help her understand the ways of the Earth Ponies better, Luna nominates Applejack to take her in for a year and teach her the ways of a modern, earth pony farmer.
Applejack and Luna protect the Dreamlands from some Lovecraftian creatures that want to get in Celestia's realm of light. And maybe they'll learn to love one another. Crossover with the Lovecraftian Dream Cycle.
Before her fall, Luna lived entire lifetimes as a mortal pony. She now wishes to do so again, and elects to spend some time with Twilight Sparkle and her friends. She never expected to fall madly in love.