With both Celestia and Luna ascending to the next plane, Twilight will be given the responsibility of Celestia, but it is up to her to decide who will take over Luna's job. But who will it be? (Just a side story I am writing while taking a break from working on my others. This is a what-if scenario taking place in the future seeing what happens if Twilight chose Applejack or Pinkie Pie and a conclusion for Spike.)